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/prisma/schema.prisma

@@ -0,0 +1,13 @@
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+// Get a free hosted Postgres database in seconds: `npx create-db`
+
+generator client {
+  provider = "prisma-client"
+  output   = "../generated/prisma"
+}
+
+datasource db {
+  provider = "postgresql"
+}