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

Prisma 7 introduced two breaking changes that conflict with
nestjs-prisma 0.27.0:

1. The `prisma-client` provider (new default in Prisma 7) outputs
   the typed client to a custom path declared in schema.prisma.
   nestjs-prisma's PrismaService extends @prisma/client's runtime,
   which boots through @prisma/client/default.js — that stub requires
   `.prisma/client/default` in a sibling node_modules tree. Only the
   legacy `prisma-client-js` generator populates that path.

2. PrismaClientOptions in Prisma 7 no longer exposes `datasourceUrl`
   nor `datasources`. Connection now requires a driver adapter
   (e.g. `@prisma/adapter-pg` for Postgres). nestjs-prisma 0.27.0
   does not pass an adapter, and `new PrismaClient(undefined)` is
   rejected with `PrismaClientInitializationError: PrismaClient needs
   to be constructed with a non-empty, valid PrismaClientOptions`.

Both quirks are Prisma 7's adapter-first architecture surfacing
through a still-Prisma-6-shaped wrapper. nestjs-prisma's peer-deps
say `^7.0.0` but its design assumes the older API. Working around
each issue separately leads further into bespoke wiring (custom
PrismaService, manual @prisma/adapter-pg setup) — bricolage on a
foundational layer.

Per CLAUDE.md ("default to stable, recognized, battle-tested
choices"), downgrade to Prisma 6.19 (still actively maintained):

- package.json: prisma + @prisma/client pinned to ^6 (resolved 6.19.3)
- apps/portal-bff/prisma/schema.prisma:
  - generator client: `prisma-client-js` (Prisma 6 default)
  - datasource db: explicit `url = env("DATABASE_URL")` (required by
    Prisma 6, was implicit in Prisma 7 via prisma.config.ts)
- apps/portal-bff/prisma.config.ts: removed (Prisma 7-only feature)
- apps/portal-bff/.gitignore: drop the now-irrelevant
  `/generated/prisma` entry (no custom output dir with prisma-client-js)

ADR-0006 ("Persistence — PostgreSQL with Prisma") is unchanged: it
specifies Prisma without pinning a version. The version choice is a
tactical detail.

Re-evaluate when nestjs-prisma releases an update aligned with
Prisma 7's adapter model.

Verified: pnpm exec prisma generate populates Prisma 6's
.prisma/client/default.{js,d.ts}; pnpm nx build portal-bff green.
Runtime serve to be re-verified locally before merge.

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")
 }

package.json

@@ -69,7 +69,7 @@
     "nx": "22.7.0",
     "postcss": "^8.5.12",
     "prettier": "^3.8.1",
-    "prisma": "^7.8.0",
+    "prisma": "^6.19.3",
     "tailwindcss": "^4.2.4",
     "ts-jest": "^29.4.0",
     "ts-node": "10.9.1",
@@ -90,7 +90,7 @@
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-express": "^11.0.0",
-    "@prisma/client": "^7.8.0",
+    "@prisma/client": "^6.19.3",
     "axios": "^1.6.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.15.1",