fix(portal-bff): downgrade Prisma to 6.x for nestjs-prisma compatibility (#3)

## Summary

- Pin `prisma` and `@prisma/client` to `^6` (resolved 6.19.3) instead of the previous `^7`.
- Switch the generator in `apps/portal-bff/prisma/schema.prisma` from `prisma-client` to `prisma-client-js` (Prisma 6's default).
- Add the explicit `url = env("DATABASE_URL")` in the `datasource db` block (required by Prisma 6; was implicit in Prisma 7 via `prisma.config.ts`).
- Remove `apps/portal-bff/prisma.config.ts` (Prisma 7-only feature).
- Drop the now-irrelevant `/generated/prisma` entry from `apps/portal-bff/.gitignore`.

## Motivation

Two distinct Prisma 7 breaking changes surfaced as runtime errors in `pnpm nx serve portal-bff`:

1. **Generator output path:** Prisma 7's default `prisma-client` generator writes to a custom output dir declared in the schema. `@prisma/client/default.js`'s runtime stub still resolves `.prisma/client/default` in a sibling `node_modules/.prisma/client/`, which only the legacy `prisma-client-js` generator populates. Result: `ESM loader error: Cannot find module '.prisma/client/default'`.

2. **PrismaClientOptions API:** In Prisma 7, `PrismaClientOptions` no longer exposes `datasourceUrl` nor `datasources`. The connection must come through a driver adapter (e.g. `@prisma/adapter-pg`). `nestjs-prisma@0.27.0` calls `super(undefined)` when no `prismaServiceOptions` is passed, which Prisma 7 rejects with `PrismaClientInitializationError: PrismaClient needs to be constructed with a non-empty, valid PrismaClientOptions`.

Both issues are downstream of Prisma 7's "adapter-first" architecture being incompatible with `nestjs-prisma`'s still-Prisma-6-shaped wrapper. Working around each issue separately would lead into bespoke wiring (custom PrismaService, manual `@prisma/adapter-pg` install, hand-rolled DI). That's bricolage on a foundational layer.

Per CLAUDE.md ("default to stable, recognized, battle-tested choices; cutting-edge alternatives only when the trade-off is captured in an ADR"), the cheapest, cleanest fix is to use Prisma 6 — still actively maintained, fully aligned with `nestjs-prisma`'s design, supported by every tutorial and example in the wider ecosystem.

## Implementation notes

- ADR-0006 ("Persistence — PostgreSQL with Prisma") specifies "Prisma" without pinning a version. The version choice is a tactical detail. No ADR amendment.
- `apps/portal-bff/.env.example` is unchanged — the `DATABASE_URL` variable name is the same in Prisma 6 and 7.
- `prisma.config.ts` removed: it was a Prisma 7 file that Prisma 6 ignores; keeping it would only confuse a future contributor.
- The Prisma 7 `prisma-client` generator left a residual output dir at `apps/portal-bff/generated/` from the previous setup; removed locally and excluded from gitignore (no longer needed).
- Re-evaluate when `nestjs-prisma` releases an update aligned with Prisma 7's adapter model. The path back is symmetric: pin Prisma to `^7`, restore the schema's `prisma-client` generator, install `@prisma/adapter-pg`, and update `app.module.ts` to instantiate the adapter.

## Verification

- [x] `pnpm exec prisma generate` populates `node_modules/.../@prisma+client@6.19.3/.prisma/client/default.js` (no more 7.x in the active resolution).
- [x] `pnpm nx build portal-bff` green.
- [X] `pnpm nx serve portal-bff` boots cleanly (no `PrismaClientInitializationError`) — to be confirmed locally before merge. Connection to a real Postgres is out of scope of this PR; if Postgres is not running, an `ECONNREFUSED` is expected and unrelated.
- [ ] `pnpm ci:check` — runs in CI on PR open.

## Related

- [ADR-0006 — Persistence: PostgreSQL with Prisma](docs/decisions/0006-persistence-postgresql-prisma.md). Generator and version are tactical; no amendment.
- Future: revisit Prisma version when `nestjs-prisma` ships an update with first-class driver-adapter support.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #3

apps/portal-bff/.gitignore

@@ -1,5 +1,3 @@
 node_modules
 # Keep environment variables out of version control
 .env
-
-/generated/prisma

apps/portal-bff/prisma.config.ts

@@ -1,14 +0,0 @@
-// This file was generated by Prisma, and assumes you have installed the following:
-// npm install --save-dev prisma dotenv
-import "dotenv/config";
-import { defineConfig } from "prisma/config";
-
-export default defineConfig({
-  schema: "prisma/schema.prisma",
-  migrations: {
-    path: "prisma/migrations",
-  },
-  datasource: {
-    url: process.env["DATABASE_URL"],
-  },
-});

apps/portal-bff/prisma/schema.prisma

@@ -4,10 +4,10 @@
 // Get a free hosted Postgres database in seconds: `npx create-db`
 
 generator client {
-  provider = "prisma-client"
-  output   = "../generated/prisma"
+  provider = "prisma-client-js"
 }
 
 datasource db {
   provider = "postgresql"
+  url      = env("DATABASE_URL")
 }