Fix CI: generate Prisma Client via postinstall

CI failed with "@prisma/client did not initialize yet" — pnpm install
never ran `prisma generate`, and `prisma migrate deploy` (unlike
`migrate dev`) doesn't do it either. Worked locally only because prior
`prisma migrate dev` runs had already generated the client as a side
effect.

Adding a postinstall script fixes it for CI and for anyone cloning the
repo fresh and running plain `pnpm install`.

apps/api/package.json

@@ -10,7 +10,8 @@
     "test": "cross-env NODE_ENV=test mocha",
     "test:bdd": "cross-env NODE_ENV=test NODE_OPTIONS=--import=tsx cucumber-js",
     "prisma:generate": "prisma generate",
-    "prisma:migrate": "prisma migrate dev"
+    "prisma:migrate": "prisma migrate dev",
+    "postinstall": "prisma generate"
   },
   "dependencies": {
     "@prisma/client": "^5.22.0",