diff --git a/README.md b/README.md
index 7899c97..077f274 100644
--- a/README.md
+++ b/README.md
@@ -11,9 +11,10 @@ Monorepo pnpm workspaces :
   `loginSchema`), types (`SafeUserProfile`), et le contrat d'erreurs (`ErrorCode`
   numérique, `ApiErrorResponse`, voir [specs/error-handling.md](specs/error-handling.md)) —
   même règles des deux côtés, pas de risque de dérive entre front et back.
-- `packages/express-tools` — outillage Express générique et réutilisable (`HttpError`,
-  `ErrorHandlerService`, `createErrorMiddleware`), séparé d'`apps/api` : pas de
-  logique métier, juste de l'infra Express.
+- `packages/express-tools` — outillage Express générique et réutilisable : `ExpressServer`
+  (init serveur, routes, middlewares), `wrapAsyncHandler`, `HttpError`,
+  `ErrorHandlerService`, `createErrorMiddleware` — séparé d'`apps/api`, pas de
+  logique métier. Détail : [specs/backend-architecture.md](specs/backend-architecture.md).
 
 `packages/shared` et `packages/express-tools` ont un vrai build (`tsc` → `dist/`,
 voir leur `package.json`) : consommés en JS compilé, pas en TS brut — nécessaire
@@ -202,6 +203,15 @@ en dur nulle part (toujours `ErrorCode.XXX`, y compris dans les mocks Cypress).
 Détail complet (schéma, exemples, comment ajouter un nouveau code d'erreur) :
 [specs/error-handling.md](specs/error-handling.md).
 
+Le profil authentifié (`requireAuth`) passe par `res.locals.userProfile`
+(typé via `AuthLocals`), pas par une augmentation du namespace global Express —
+voir [specs/backend-architecture.md](specs/backend-architecture.md) pour le détail
+et le pourquoi.
+
+`packages/shared` fournit aussi `assertIsNever` (vérification d'exhaustivité de
+switch/if-chain sur une union, erreur de **compilation** si un cas est oublié) —
+voir [specs/backend-architecture.md](specs/backend-architecture.md#packagesshared--assertisnever).
+
 ## i18n
 
 **i18next** + **react-i18next** — tout le texte affiché (formulaires, boutons,
diff --git a/apps/api/package.json b/apps/api/package.json
index 530d581..f73a193 100644
--- a/apps/api/package.json
+++ b/apps/api/package.json
@@ -18,8 +18,6 @@
     "@batch-cooking/shared": "workspace:*",
     "@prisma/client": "^5.22.0",
     "argon2": "0.31.2",
-    "cookie-parser": "^1.4.7",
-    "cors": "^2.8.6",
     "dotenv": "^16.4.5",
     "express": "^4.21.1",
     "jsonwebtoken": "^9.0.3",
@@ -28,8 +26,6 @@
   "devDependencies": {
     "@cucumber/cucumber": "^13.2.1",
     "@faker-js/faker": "^10.6.0",
-    "@types/cookie-parser": "^1.4.10",
-    "@types/cors": "^2.8.19",
     "@types/express": "^4.17.21",
     "@types/jsonwebtoken": "^9.0.10",
     "@types/node": "^22.9.0",
diff --git a/apps/api/src/app.ts b/apps/api/src/app.ts
index db65ad0..a950827 100644
--- a/apps/api/src/app.ts
+++ b/apps/api/src/app.ts
@@ -1,37 +1,34 @@
-import { createErrorMiddleware, errorHandlerService } from "@batch-cooking/express-tools";
+import {
+  ExpressServer,
+  createErrorMiddleware,
+  errorHandlerService,
+} from "@batch-cooking/express-tools";
 import { ErrorCode } from "@batch-cooking/shared";
-import cookieParser from "cookie-parser";
-import cors from "cors";
-import express, { type Request, type Response } from "express";
+import type { Express, Request, Response } from "express";
 import { env } from "./config/env.js";
 import { authRouter } from "./modules/auth/auth.routes.js";
 
 /**
- * Builds a fresh Express application instance (no shared mutable state
- * between calls — used both by the real server entrypoint and by tests,
- * which each get their own app via supertest).
- *
- * Feature modules are mounted under `/`-prefixed routers as specs land;
- * `auth` is the first one (login page / profile creation).
+ * Builds the API's `ExpressServer`: standard middleware, routes, and the
+ * final error handler, in that order. Returns the `ExpressServer` wrapper
+ * (not just the raw Express app) so `server.ts` can call `.listen()` on
+ * it — {@link createApp} below is the thinner entry point that exposes
+ * just the raw `Express` instance, for test tooling (supertest) that
+ * expects one.
  */
-export function createApp() {
-  const app = express();
+export function createServer(): ExpressServer {
+  const server = new ExpressServer();
+  server.setupCore({ corsOrigin: env.CORS_ORIGIN });
 
-  // Frontend and API run on different origins — `credentials: true` is
-  // required for the httpOnly session cookie to be sent/received.
-  app.use(cors({ origin: env.CORS_ORIGIN, credentials: true }));
-  app.use(express.json());
-  app.use(cookieParser());
-
-  app.get("/health", (_req: Request, res: Response) => {
+  server.addRoute("get", "/health", (_req: Request, res: Response) => {
     res.status(200).json({ status: "ok" });
   });
 
-  app.use("/auth", authRouter);
+  server.mountRouter("/auth", authRouter);
 
   // No route matched — same shape as every other error response, via the
   // shared ErrorCode contract, so clients never special-case 404s.
-  app.use((_req: Request, res: Response) => {
+  server.addMiddleware((_req: Request, res: Response) => {
     res.status(404).json({ code: ErrorCode.NOT_FOUND, message: "Not found" });
   });
 
@@ -39,7 +36,17 @@ export function createApp() {
   // the app ends up here. All the "what status/body does this error map
   // to" logic lives in ErrorHandlerService, from @batch-cooking/express-tools
   // — this stays a thin adapter.
-  app.use(createErrorMiddleware(errorHandlerService));
+  server.setErrorHandler(createErrorMiddleware(errorHandlerService));
 
-  return app;
+  return server;
+}
+
+/**
+ * Builds a fresh Express application instance (no shared mutable state
+ * between calls — used both indirectly by the real server entrypoint
+ * (`server.ts`, via {@link createServer}) and directly by tests, which
+ * each get their own app via supertest).
+ */
+export function createApp(): Express {
+  return createServer().instance;
 }
diff --git a/apps/api/src/middlewares/require-auth.ts b/apps/api/src/middlewares/require-auth.ts
index dacb37a..31c33ca 100644
--- a/apps/api/src/middlewares/require-auth.ts
+++ b/apps/api/src/middlewares/require-auth.ts
@@ -1,10 +1,21 @@
 import { HttpError } from "@batch-cooking/express-tools";
-import { ErrorCode } from "@batch-cooking/shared";
+import { ErrorCode, type SafeUserProfile } from "@batch-cooking/shared";
 import type { NextFunction, Request, Response } from "express";
 import { env } from "../config/env.js";
 import { prisma } from "../db/prisma.js";
 import { verifyAuthToken } from "../lib/jwt.js";
 
+/**
+ * Shape of `res.locals` once {@link requireAuth} has run successfully. Type
+ * a route handler's response as `Response<unknown, AuthLocals>` (see
+ * `auth.routes.ts`'s `/me` handler) to read `res.locals.userProfile` fully
+ * typed, no cast needed.
+ */
+export interface AuthLocals {
+  /** The authenticated profile, resolved from the session cookie's JWT. */
+  userProfile: SafeUserProfile;
+}
+
 /**
  * Express middleware guarding routes that require an authenticated
  * profile. Reads the session cookie, verifies the JWT, and re-checks
@@ -12,14 +23,24 @@ import { verifyAuthToken } from "../lib/jwt.js";
  * invalidated server-side (e.g. on password change / logout-everywhere,
  * once that feature exists) despite carrying no server-side session.
  *
- * On success, attaches the resolved profile to `req.userProfile` (see
- * `src/types/express-request.augment.ts`) for downstream handlers to use.
+ * On success, attaches the resolved profile to `res.locals.userProfile`
+ * (typed via {@link AuthLocals}) for downstream handlers to use.
+ * Deliberately `res.locals` rather than augmenting Express's global
+ * `Request` type via `declare global`: `res.locals` is Express's own
+ * built-in mechanism for exactly this (passing data from a middleware to
+ * the next handler), typed per-route through a generic parameter — no
+ * project-wide ambient augmentation silently changing every `Request` in
+ * the codebase, whether or not it went through this middleware.
  *
  * @throws {HttpError} `401 NOT_AUTHENTICATED` for any failure — missing
  *   cookie, malformed/expired JWT, unknown profile, or stale tokenVersion.
  *   Never distinguishes the reason to the client.
  */
-export async function requireAuth(req: Request, _res: Response, next: NextFunction) {
+export async function requireAuth(
+  req: Request,
+  res: Response<unknown, AuthLocals>,
+  next: NextFunction,
+) {
   try {
     const token = req.cookies?.[env.AUTH_COOKIE_NAME];
     if (typeof token !== "string") {
@@ -34,7 +55,7 @@ export async function requireAuth(req: Request, _res: Response, next: NextFuncti
     }
 
     const { passwordHash: _passwordHash, ...safeProfile } = profile;
-    req.userProfile = safeProfile;
+    res.locals.userProfile = safeProfile;
     next();
   } catch (err) {
     if (err instanceof HttpError) {
diff --git a/apps/api/src/modules/auth/auth.routes.ts b/apps/api/src/modules/auth/auth.routes.ts
index 83760a2..044a8ab 100644
--- a/apps/api/src/modules/auth/auth.routes.ts
+++ b/apps/api/src/modules/auth/auth.routes.ts
@@ -1,8 +1,9 @@
+import { wrapAsyncHandler } from "@batch-cooking/express-tools";
 import { loginSchema, signupSchema } from "@batch-cooking/shared";
 import { Router } from "express";
-import type { CookieOptions } from "express";
+import type { CookieOptions, Response } from "express";
 import { env } from "../../config/env.js";
-import { requireAuth } from "../../middlewares/require-auth.js";
+import { type AuthLocals, requireAuth } from "../../middlewares/require-auth.js";
 import { login, signup } from "./auth.service.js";
 
 /** Router mounted at `/auth` in app.ts — signup, login, logout, current-profile. */
@@ -23,29 +24,31 @@ const cookieOptions: CookieOptions = {
   maxAge: SEVEN_DAYS_MS,
 };
 
-/** Creates a profile (+ its household) and logs the new user in immediately. */
-authRouter.post("/signup", async (req, res, next) => {
-  try {
+/**
+ * Creates a profile (+ its household) and logs the new user in
+ * immediately. `wrapAsyncHandler` forwards a thrown/rejected error to
+ * Express's error middleware automatically — no manual try/catch needed.
+ */
+authRouter.post(
+  "/signup",
+  wrapAsyncHandler(async (req, res) => {
     const input = signupSchema.parse(req.body);
     const { profile, token } = await signup(input);
     res.cookie(env.AUTH_COOKIE_NAME, token, cookieOptions);
     res.status(201).json(profile);
-  } catch (err) {
-    next(err);
-  }
-});
+  }),
+);
 
 /** Verifies credentials and starts a new session. */
-authRouter.post("/login", async (req, res, next) => {
-  try {
+authRouter.post(
+  "/login",
+  wrapAsyncHandler(async (req, res) => {
     const input = loginSchema.parse(req.body);
     const { profile, token } = await login(input);
     res.cookie(env.AUTH_COOKIE_NAME, token, cookieOptions);
     res.status(200).json(profile);
-  } catch (err) {
-    next(err);
-  }
-});
+  }),
+);
 
 /** Ends the current session by clearing the cookie. Stateless JWT, so there's nothing to revoke server-side (yet — see tokenVersion). */
 authRouter.post("/logout", (_req, res) => {
@@ -54,6 +57,6 @@ authRouter.post("/logout", (_req, res) => {
 });
 
 /** Returns the currently authenticated profile. Behind requireAuth — 401s if there's no valid session. */
-authRouter.get("/me", requireAuth, (req, res) => {
-  res.status(200).json(req.userProfile);
+authRouter.get("/me", requireAuth, (_req, res: Response<unknown, AuthLocals>) => {
+  res.status(200).json(res.locals.userProfile);
 });
diff --git a/apps/api/src/modules/auth/auth.service.ts b/apps/api/src/modules/auth/auth.service.ts
index ec6b740..cfd5b7b 100644
--- a/apps/api/src/modules/auth/auth.service.ts
+++ b/apps/api/src/modules/auth/auth.service.ts
@@ -11,7 +11,9 @@ type SafeProfile = Omit<UserProfile, "passwordHash">;
 
 /** Result of a successful signup/login: the safe profile plus the signed session JWT to set as a cookie. */
 interface AuthResult {
+  /** The authenticated profile, safe to hand back to the client. */
   profile: SafeProfile;
+  /** Signed session JWT — the caller sets this as the session cookie's value. */
   token: string;
 }
 
diff --git a/apps/api/src/server.ts b/apps/api/src/server.ts
index 89dabe6..f7753cc 100644
--- a/apps/api/src/server.ts
+++ b/apps/api/src/server.ts
@@ -1,8 +1,8 @@
-import { createApp } from "./app.js";
+import { createServer } from "./app.js";
 import { env } from "./config/env.js";
 
-const app = createApp();
+const server = createServer();
 
-app.listen(env.PORT, () => {
+server.listen(env.PORT, () => {
   console.log(`API listening on http://localhost:${env.PORT}`);
 });
diff --git a/apps/api/src/types/express-request.augment.ts b/apps/api/src/types/express-request.augment.ts
deleted file mode 100644
index 3c20ed9..0000000
--- a/apps/api/src/types/express-request.augment.ts
+++ /dev/null
@@ -1,18 +0,0 @@
-import type { UserProfile } from "@prisma/client";
-
-// Module augmentation: adds a `userProfile` field to Express's Request type
-// so `requireAuth` can attach the authenticated profile and downstream
-// handlers get it fully typed, without an `as` cast at every call site.
-//
-// Deliberately a plain .ts file, not a .d.ts: `declare global` augmentation
-// works identically either way as long as the file has a top-level import
-// (which makes it a module TS will include in the program) — no need for
-// the .d.ts extension just for this.
-declare global {
-  namespace Express {
-    interface Request {
-      /** Set by requireAuth once the session cookie's JWT has been verified. */
-      userProfile?: Omit<UserProfile, "passwordHash">;
-    }
-  }
-}
diff --git a/packages/express-tools/package.json b/packages/express-tools/package.json
index 27d3d09..68b90d3 100644
--- a/packages/express-tools/package.json
+++ b/packages/express-tools/package.json
@@ -17,12 +17,16 @@
     "postinstall": "tsc -p tsconfig.json"
   },
   "devDependencies": {
+    "@types/cookie-parser": "^1.4.10",
+    "@types/cors": "^2.8.19",
     "@types/express": "^4.17.21",
     "@types/node": "^22.9.0",
     "typescript": "^5.7.2"
   },
   "dependencies": {
     "@batch-cooking/shared": "workspace:*",
+    "cookie-parser": "^1.4.7",
+    "cors": "^2.8.6",
     "express": "^4.21.1",
     "zod": "^3.25.76"
   }
diff --git a/packages/express-tools/src/async-handler.ts b/packages/express-tools/src/async-handler.ts
new file mode 100644
index 0000000..1dcd692
--- /dev/null
+++ b/packages/express-tools/src/async-handler.ts
@@ -0,0 +1,34 @@
+import type { NextFunction, Request, RequestHandler, Response } from "express";
+
+/**
+ * An Express route handler whose body is `async` (returns a `Promise`).
+ * Only `ResBody`/`Locals` are made generic (what this codebase actually
+ * varies per-route) — params/request-body/query stay at Express's own
+ * internal defaults, same as an unparameterized `Request`.
+ */
+export type AsyncRequestHandler<
+  ResBody = unknown,
+  Locals extends Record<string, unknown> = Record<string, unknown>,
+> = (req: Request, res: Response<ResBody, Locals>, next: NextFunction) => Promise<void>;
+
+/**
+ * Wraps an async Express handler so a thrown error or rejected promise is
+ * forwarded to `next(err)` automatically. Without this, an unhandled
+ * rejection inside an `async` route handler never reaches Express's error
+ * middleware — every route ends up needing its own `try { ... } catch (err)
+ * { next(err); }` boilerplate, which this removes.
+ *
+ * @example
+ * router.post("/signup", wrapAsyncHandler(async (req, res) => {
+ *   const profile = await signup(req.body);
+ *   res.status(201).json(profile);
+ * }));
+ */
+export function wrapAsyncHandler<
+  ResBody = unknown,
+  Locals extends Record<string, unknown> = Record<string, unknown>,
+>(handler: AsyncRequestHandler<ResBody, Locals>): RequestHandler {
+  return (req, res, next) => {
+    handler(req, res as Response<ResBody, Locals>, next).catch(next);
+  };
+}
diff --git a/packages/express-tools/src/express-server.ts b/packages/express-tools/src/express-server.ts
new file mode 100644
index 0000000..1e95541
--- /dev/null
+++ b/packages/express-tools/src/express-server.ts
@@ -0,0 +1,96 @@
+import cookieParser from "cookie-parser";
+import cors from "cors";
+import express, {
+  type ErrorRequestHandler,
+  type Express,
+  type RequestHandler,
+  type Router,
+} from "express";
+
+/** HTTP verbs {@link ExpressServer.addRoute} accepts. */
+export type HttpMethod = "get" | "post" | "put" | "patch" | "delete";
+
+/** Options for {@link ExpressServer.setupCore}. */
+export interface ExpressServerCoreOptions {
+  /** Origin allowed by CORS — must match wherever the frontend is served from. */
+  corsOrigin: string;
+}
+
+/**
+ * Thin wrapper around an Express application: bundles the common
+ * "set up the standard middleware stack, register routes without
+ * duplicates, wire the error handler, start listening" concerns behind a
+ * small typed API, instead of every service in the monorepo repeating the
+ * same raw `express()` setup by hand.
+ *
+ * Framework-specific on purpose — unlike `ErrorHandlerService` (which has
+ * no Express dependency at all), this class *is* the Express integration
+ * layer. Business/domain code should never import `express` directly;
+ * it goes through this instead.
+ */
+export class ExpressServer {
+  /** The underlying Express application. */
+  private readonly app: Express;
+  /** Tracks `"METHOD path"` keys already registered via {@link addRoute}, to warn instead of silently double-registering a route. */
+  private readonly registeredRoutes = new Set<string>();
+
+  public constructor() {
+    this.app = express();
+  }
+
+  /** The underlying Express application — needed by test tooling (e.g. supertest) that expects a raw `Express` instance. */
+  public get instance(): Express {
+    return this.app;
+  }
+
+  /**
+   * Registers the standard middleware stack every service in this
+   * monorepo needs: CORS (with credentials, for the session cookie),
+   * JSON body parsing, and cookie parsing. Call once, before registering
+   * any route.
+   */
+  public setupCore(options: ExpressServerCoreOptions): void {
+    this.app.use(cors({ origin: options.corsOrigin, credentials: true }));
+    this.app.use(express.json());
+    this.app.use(cookieParser());
+  }
+
+  /** Registers a middleware that runs on every request (e.g. logging, a catch-all 404 handler). */
+  public addMiddleware(middleware: RequestHandler): void {
+    this.app.use(middleware);
+  }
+
+  /**
+   * Registers the final Express error-handling middleware (the 4-argument
+   * form). Must be added last — Express only treats a middleware as an
+   * error handler by its arity, and only the last matching one runs.
+   */
+  public setErrorHandler(middleware: ErrorRequestHandler): void {
+    this.app.use(middleware);
+  }
+
+  /** Mounts a whole `express.Router` under a base path (e.g. `mountRouter("/auth", authRouter)`). */
+  public mountRouter(basePath: string, router: Router): void {
+    this.app.use(basePath, router);
+  }
+
+  /**
+   * Registers a single route with its handler(s). Warns and skips instead
+   * of registering if the same method+path was already added — catches a
+   * copy-paste mistake at startup instead of silently shadowing a route.
+   */
+  public addRoute(method: HttpMethod, path: string, ...handlers: RequestHandler[]): void {
+    const key = `${method.toUpperCase()} ${path}`;
+    if (this.registeredRoutes.has(key)) {
+      console.warn(`[ExpressServer] Route already registered, skipping: ${key}`);
+      return;
+    }
+    this.registeredRoutes.add(key);
+    this.app[method](path, ...handlers);
+  }
+
+  /** Starts listening on the given port. `onListening` is called once the server is up (e.g. to log the URL). */
+  public listen(port: number, onListening?: () => void): void {
+    this.app.listen(port, onListening);
+  }
+}
diff --git a/packages/express-tools/src/index.ts b/packages/express-tools/src/index.ts
index 69abfd0..cde2fbd 100644
--- a/packages/express-tools/src/index.ts
+++ b/packages/express-tools/src/index.ts
@@ -2,7 +2,16 @@
 // Express app in this monorepo (currently apps/api). Generic HTTP/Express
 // infrastructure lives here — domain-specific code (auth, business logic)
 // stays in the consuming app.
+//
+// Note the split: ErrorHandlerService (error-handler.service.ts) has zero
+// dependency on Express — it's a plain "map an error to {status, body}"
+// service that would work identically behind Fastify or any other
+// framework. ExpressServer and createErrorMiddleware are the actual
+// Express-specific layer, adapting framework-agnostic pieces (like
+// ErrorHandlerService) onto Express's API.
 
+export * from "./async-handler.js";
 export * from "./error-handler.service.js";
 export * from "./error-middleware.js";
+export * from "./express-server.js";
 export * from "./http-error.js";
diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index b0017b4..98ed583 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -5,4 +5,5 @@
 
 export * from "./errors/error-codes.js";
 export * from "./schemas/auth.js";
+export * from "./tools/assert-is-never.js";
 export * from "./types/user-profile.js";
diff --git a/packages/shared/src/tools/assert-is-never.ts b/packages/shared/src/tools/assert-is-never.ts
new file mode 100644
index 0000000..8e59742
--- /dev/null
+++ b/packages/shared/src/tools/assert-is-never.ts
@@ -0,0 +1,35 @@
+/**
+ * Exhaustiveness check for a `switch`/`if`-chain over a union type. Call
+ * this in the `default` case (or final `else`) with the value being
+ * switched on: if every member of the union has been handled by an
+ * earlier branch, TypeScript narrows that value to `never` there, and the
+ * call type-checks. If a new member is later added to the union and a
+ * branch is forgotten, `value` is no longer `never` at that point — the
+ * call becomes a **compile error**, catching the missing case before it
+ * ships, instead of silently falling through at runtime.
+ *
+ * @example
+ * type Shape = { kind: "circle"; radius: number } | { kind: "square"; side: number };
+ *
+ * function area(shape: Shape): number {
+ *   switch (shape.kind) {
+ *     case "circle":
+ *       return Math.PI * shape.radius ** 2;
+ *     case "square":
+ *       return shape.side ** 2;
+ *     default:
+ *       // Compile error here if a new `Shape` variant is added without a
+ *       // matching case above — `shape` wouldn't be `never` anymore.
+ *       return assertIsNever(shape);
+ *   }
+ * }
+ *
+ * @param value - The value that should be `never` at this point (compile-time check).
+ * @param message - Optional custom error message; defaults to including the offending value.
+ * @throws Always throws — this is also a real runtime safety net for a
+ *   value that reaches here despite the type system (e.g. unvalidated
+ *   external data cast to the union type).
+ */
+export function assertIsNever(value: never, message?: string): never {
+  throw new Error(message ?? `Unexpected value: ${JSON.stringify(value)}`);
+}
diff --git a/packages/shared/src/types/user-profile.ts b/packages/shared/src/types/user-profile.ts
index f4c0056..c221579 100644
--- a/packages/shared/src/types/user-profile.ts
+++ b/packages/shared/src/types/user-profile.ts
@@ -7,8 +7,11 @@
 export interface SafeUserProfile {
   /** Primary key. */
   id: number;
+  /** First name. */
   firstName: string;
+  /** Last name. */
   lastName: string;
+  /** Email address — unique, used as the login identifier. */
   email: string;
   /** Incremented server-side to invalidate previously-issued JWTs (e.g. on password change). Not used directly by the client. */
   tokenVersion: number;
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 5586805..9cae0c7 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -29,12 +29,6 @@ importers:
       argon2:
         specifier: 0.31.2
         version: 0.31.2
-      cookie-parser:
-        specifier: ^1.4.7
-        version: 1.4.7
-      cors:
-        specifier: ^2.8.6
-        version: 2.8.6
       dotenv:
         specifier: ^16.4.5
         version: 16.6.1
@@ -54,12 +48,6 @@ importers:
       '@faker-js/faker':
         specifier: ^10.6.0
         version: 10.6.0
-      '@types/cookie-parser':
-        specifier: ^1.4.10
-        version: 1.4.10(@types/express@4.17.25)
-      '@types/cors':
-        specifier: ^2.8.19
-        version: 2.8.19
       '@types/express':
         specifier: ^4.17.21
         version: 4.17.25
@@ -151,6 +139,12 @@ importers:
       '@batch-cooking/shared':
         specifier: workspace:*
         version: link:../shared
+      cookie-parser:
+        specifier: ^1.4.7
+        version: 1.4.7
+      cors:
+        specifier: ^2.8.6
+        version: 2.8.6
       express:
         specifier: ^4.21.1
         version: 4.22.2
@@ -158,6 +152,12 @@ importers:
         specifier: ^3.25.76
         version: 3.25.76
     devDependencies:
+      '@types/cookie-parser':
+        specifier: ^1.4.10
+        version: 1.4.10(@types/express@4.17.25)
+      '@types/cors':
+        specifier: ^2.8.19
+        version: 2.8.19
       '@types/express':
         specifier: ^4.17.21
         version: 4.17.25
diff --git a/specs/backend-architecture.md b/specs/backend-architecture.md
new file mode 100644
index 0000000..a7f5b4f
--- /dev/null
+++ b/specs/backend-architecture.md
@@ -0,0 +1,136 @@
+# Architecture backend — Projet Batch-cooking
+
+> Documentation de l'organisation d'`apps/api` et de l'outillage partagé
+> (`packages/express-tools`, `packages/shared`).
+
+---
+
+## `packages/express-tools` — outillage Express générique
+
+Package séparé, réutilisable par n'importe quel service Express du monorepo (pas
+seulement `apps/api`) : pas de logique métier, juste de l'infra Express.
+
+### `ExpressServer` — init serveur, routes, middlewares
+
+Enveloppe une application Express derrière une API typée, au lieu que chaque
+service refasse le même `express()` à la main :
+
+```ts
+const server = new ExpressServer();
+server.setupCore({ corsOrigin: env.CORS_ORIGIN }); // cors + json + cookie-parser
+server.addRoute("get", "/health", (_req, res) => res.status(200).json({ status: "ok" }));
+server.mountRouter("/auth", authRouter);
+server.addMiddleware(notFoundHandler);
+server.setErrorHandler(createErrorMiddleware(errorHandlerService));
+server.listen(port, () => console.log(`Listening on ${port}`));
+```
+
+- `setupCore(options)` — middleware stack commun (CORS avec credentials, JSON,
+  cookies).
+- `addRoute(method, path, ...handlers)` — enregistre une route ; avertit et
+  ignore au lieu d'écraser silencieusement si la même route (méthode + chemin)
+  est déjà enregistrée.
+- `addMiddleware` / `mountRouter` / `setErrorHandler` — ajout de middleware
+  générique, montage d'un `Router` complet, middleware d'erreur final (4
+  arguments — doit être ajouté en dernier).
+- `.instance` — l'app Express brute, nécessaire pour les outils de test
+  (supertest) qui attendent une instance `Express`, pas le wrapper.
+- `.listen(port, onListening?)` — démarre le serveur.
+
+`apps/api/src/app.ts` expose deux fonctions : `createServer(): ExpressServer`
+(utilisée par `server.ts`, qui appelle `.listen()`) et `createApp(): Express`
+(= `createServer().instance`, utilisée par les tests).
+
+### `wrapAsyncHandler` — plus de try/catch répété dans les routes
+
+```ts
+router.post("/signup", wrapAsyncHandler(async (req, res) => {
+  const profile = await signup(req.body); // une erreur/rejet ici va automatiquement à next()
+  res.status(201).json(profile);
+}));
+```
+
+Sans ça, une exception dans un handler `async` ne remonte jamais tout seule au
+middleware d'erreur d'Express — chaque route devait faire son propre
+`try { ... } catch (err) { next(err); }`. `wrapAsyncHandler` l'automatise.
+
+### `HttpError` / `ErrorHandlerService` / `createErrorMiddleware`
+
+Voir [error-handling.md](./error-handling.md) pour le détail. À noter :
+`ErrorHandlerService` **n'a aucune dépendance à Express** — c'est un service
+générique `erreur → { status, body }` qui fonctionnerait à l'identique derrière
+Fastify ou n'importe quel autre framework. `ExpressServer` et
+`createErrorMiddleware` sont la vraie couche Express : elles adaptent des
+pièces indépendantes du framework (comme `ErrorHandlerService`) à l'API
+d'Express. C'est pour ça que `ErrorHandlerService` n'importe jamais `express`.
+
+---
+
+## Auth : `res.locals`, pas d'augmentation du namespace Express
+
+`requireAuth` (`apps/api/src/middlewares/require-auth.ts`) attache le profil
+authentifié à **`res.locals.userProfile`**, typé via l'interface `AuthLocals` :
+
+```ts
+export interface AuthLocals {
+  userProfile: SafeUserProfile;
+}
+
+export async function requireAuth(req: Request, res: Response<unknown, AuthLocals>, next: NextFunction) {
+  // ...
+  res.locals.userProfile = safeProfile;
+  next();
+}
+```
+
+Un handler derrière ce middleware type sa réponse `Response<unknown, AuthLocals>`
+et lit `res.locals.userProfile` sans cast :
+
+```ts
+authRouter.get("/me", requireAuth, (_req, res: Response<unknown, AuthLocals>) => {
+  res.status(200).json(res.locals.userProfile);
+});
+```
+
+**Pourquoi pas `declare global { namespace Express { interface Request {...} } }`**
+(l'approche initialement utilisée, retirée depuis) : `res.locals` est le
+mécanisme natif d'Express prévu exactement pour ça (faire passer des données
+d'un middleware au handler suivant), typé par route via un paramètre
+générique — pas une augmentation globale et permanente qui change
+silencieusement le type de **toutes** les `Request` du projet, qu'elles soient
+passées par ce middleware ou non.
+
+---
+
+## `packages/shared` — `assertIsNever`
+
+`packages/shared/src/tools/assert-is-never.ts` — vérification d'exhaustivité
+pour un `switch`/`if`-chain sur une union :
+
+```ts
+switch (shape.kind) {
+  case "circle": return Math.PI * shape.radius ** 2;
+  case "square": return shape.side ** 2;
+  default: return assertIsNever(shape); // erreur de compilation si un cas manque
+}
+```
+
+Si un membre de l'union n'est pas traité par une branche précédente, `shape`
+n'est plus de type `never` au niveau du `default` → **erreur de compilation**
+(vérifié : `tsc` rejette bien un cas manquant). Lève aussi une vraie erreur au
+runtime, en filet de sécurité si une valeur invalide échappe au système de
+types (ex. donnée externe non validée).
+
+Pas encore de point d'usage réel dans le code métier actuel (aucun
+switch/if-chain exhaustif sur une union n'existe encore) — prêt à l'emploi dès
+qu'un cas s'y prête (le module « Calcul batch-cooking » ou le pipeline d'import
+de recette, tous deux encore à construire, en auront probablement).
+
+---
+
+## Pas de fichiers `.d.ts` écrits à la main
+
+Voir [frontend-architecture.md](./frontend-architecture.md#note-sur-les-fichiers-dts)
+pour le détail côté `apps/web`. Côté `apps/api` : aucune augmentation de type
+globale (`declare global`) n'est utilisée — voir la section `res.locals`
+ci-dessus, qui est précisément ce qui aurait nécessité ce genre de fichier.
diff --git a/specs/error-handling.md b/specs/error-handling.md
index f9a2303..b4982a4 100644
--- a/specs/error-handling.md
+++ b/specs/error-handling.md
@@ -97,10 +97,12 @@ Pour ajouter un nouveau cas d'erreur :
 
 ---
 
-## `packages/express-tools` — outillage Express générique
+## `packages/express-tools` — les pièces liées aux erreurs
 
-Séparé d'`apps/api` volontairement : ce sont des briques génériques (n'importe quel
-service Express du monorepo pourrait les utiliser), pas de logique métier.
+`packages/express-tools` contient aussi `ExpressServer` (init serveur,
+enregistrement de routes/middlewares) et `wrapAsyncHandler` — voir
+[backend-architecture.md](./backend-architecture.md) pour le détail complet du
+package. Les pièces qui concernent spécifiquement les erreurs :
 
 - **`http-error.ts`** — `HttpError` : erreur typée portant `status` (code HTTP) et
   `code` (`ErrorCode`). C'est ce que lèvent les routes/services au lieu de
@@ -109,9 +111,11 @@ service Express du monorepo pourrait les utiliser), pas de logique métier.
   transformer n'importe quelle erreur JS (`ZodError`, `HttpError`, n'importe quoi
   d'autre) en `{ status, body }`. Le cas générique (`INTERNAL_ERROR`, 500) logue
   l'erreur côté serveur sans jamais exposer de détail interne au client.
+  **N'importe pas `express`** — c'est un service générique, indépendant du
+  framework HTTP, qui fonctionnerait à l'identique derrière Fastify ou autre.
 - **`error-middleware.ts`** — `createErrorMiddleware(service)` : construit le
   middleware d'erreur Express (signature à 4 arguments) à partir du service —
-  adaptateur fin, aucune logique de mapping n'y vit.
+  c'est LUI la vraie couche Express, `ErrorHandlerService` reste agnostique.
 
 Build réel (`tsc` → `dist/`, comme `packages/shared`) : consommé en JS compilé,
 pas en TS brut — voir la note dans
@@ -120,9 +124,10 @@ sur pourquoi ça compte pour un runtime Node pur (Docker).
 
 ## Côté API (`apps/api`)
 
-- **`app.ts`** — le middleware d'erreur final est
-  `app.use(createErrorMiddleware(errorHandlerService))` ; aucune logique de
-  mapping n'y vit directement, tout est dans `express-tools`.
+- **`app.ts`** — le middleware d'erreur final est enregistré via
+  `server.setErrorHandler(createErrorMiddleware(errorHandlerService))` (voir
+  [backend-architecture.md](./backend-architecture.md) pour `ExpressServer`) ;
+  aucune logique de mapping n'y vit directement, tout est dans `express-tools`.
 - Les modules métier (`modules/auth/auth.service.ts`, `middlewares/require-auth.ts`)
   importent `HttpError` depuis `@batch-cooking/express-tools` et `ErrorCode` depuis
   `@batch-cooking/shared`.
diff --git a/specs/frontend-architecture.md b/specs/frontend-architecture.md
index 81cef66..4348234 100644
--- a/specs/frontend-architecture.md
+++ b/specs/frontend-architecture.md
@@ -116,10 +116,10 @@ Aucun fichier `.d.ts` écrit à la main dans `apps/web` : le
 `tsconfig.app.json` — même effet (typage de `import.meta.env`, imports d'assets),
 sans fichier dédié.
 
-Même logique côté `apps/api` : l'augmentation du type `Express.Request` (pour
-`req.userProfile`) vit dans `src/types/express-request.augment.ts`, un fichier
-`.ts` classique (pas `.d.ts`) — une augmentation `declare global` fonctionne
-identiquement dans les deux, tant que le fichier a un import qui en fait un module.
+Côté `apps/api`, aucune augmentation de type globale n'est utilisée du tout — voir
+[backend-architecture.md](./backend-architecture.md#auth--reslocals-pas-daugmentation-du-namespace-express)
+: le profil authentifié passe par `res.locals` (mécanisme natif d'Express), pas
+par un `declare global` sur `Express.Request`.
 
 ---