chore: wire PostgreSQL + Prisma per ADR-0006

Add Prisma 7 + nestjs-prisma. The schema lives at
apps/portal-bff/prisma/schema.prisma with provider postgresql; the new
prisma-client generator (Prisma 7 default) outputs the typed client to
apps/portal-bff/generated/prisma/ which is gitignored.

apps/portal-bff/src/app/app.module.ts imports PrismaModule.forRoot
({ isGlobal: true }) so PrismaService is injectable across the BFF
without per-module imports.

apps/portal-bff/.env.example documents DATABASE_URL with a local-dev
default, plus a forward list of env vars introduced by upcoming phases
and ADRs (auth, sessions, MFA, observability, audit, downstream APIs)
- catalog reference, not implementation. The actual .env stays
gitignored at both repo root and app levels.

prisma.config.ts (Prisma 7's TypeScript config) is committed; it loads
DATABASE_URL via dotenv. Schema and migrations paths are pinned to
prisma/ relative to the bff app.

PostgreSQL provisioning, RLS policies for the dual-audience design,
the dedicated audit schema with role grants (audit_owner / audit_writer
/ audit_reader / audit_archiver per ADR-0013), and column-level
encryption for L3-scoped data are out of scope of this commit -
they belong with the future on-prem infrastructure ADR.

apps/portal-bff/src/app/app.module.ts

@@ -1,9 +1,10 @@
 import { Module } from '@nestjs/common';
+import { PrismaModule } from 'nestjs-prisma';
 import { AppController } from './app.controller';
 import { AppService } from './app.service';
 
 @Module({
-  imports: [],
+  imports: [PrismaModule.forRoot({ isGlobal: true })],
   controllers: [AppController],
   providers: [AppService],
 })