chore(prisma): point root postinstall to BFF schema (#9)

## Summary
`@prisma/client`'s postinstall hook runs `prisma generate` from the workspace root and didn't find a schema there, so the typed client was never generated. Fine today (schema has no models, no runtime usage), but a trap as soon as the first model lands.

Declare `package.json#prisma.schema` so the postinstall locates `apps/portal-bff/prisma/schema.prisma` and generates the typed client on every `pnpm install`.

Verified locally: postinstall now logs `✔ Generated Prisma Client (v6.19.3)`.

## Notes
- Prisma 6.19 emits a deprecation warning suggesting migration to `prisma.config.ts`. That format is freshly out of Early Access and the migration is small but non-trivial — folding it into the future Prisma 7 upgrade (which itself depends on `nestjs-prisma` catching up and will get its own ADR).

## Test plan
- [ ] CI green on this PR.
- [ ] In the `Run pnpm install --frozen-lockfile` log of the `check` job, the warning `prisma:warn We could not find your Prisma schema` is gone, replaced with `✔ Generated Prisma Client`.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #9

package.json

@@ -32,6 +32,9 @@
       "unrs-resolver"
     ]
   },
+  "prisma": {
+    "schema": "apps/portal-bff/prisma/schema.prisma"
+  },
   "devDependencies": {
     "@analogjs/vite-plugin-angular": "~2.1.2",
     "@analogjs/vitest-angular": "~2.1.2",