feat(portal-bff): wire ADR-0013 audit pipeline to the auth lifecycle (#120)

## Summary

Wires the audit pipeline (ADR-0013) to the auth lifecycle. The foundation was already in place (Prisma `AuditEvent` model, Postgres roles + grants, `AuditWriter.recordEvent` with `SET LOCAL ROLE audit_writer`); this PR layers a typed event surface and emits the first four events on real code paths.

### What lands

- **Typed methods on `AuditWriter`**: `signIn`, `signInFailed`, `signOut`, `sessionExpired`. Callers pass the raw Entra `oid`; hashing happens inside the writer so the salt never leaves the audit module. ADR-0013 explicitly defers adding these typed methods "as the matching feature ships" — auth has shipped, so we add the four events tied to code paths that exist today.
- **`HashUserIdService`** — reads `LOG_USER_ID_SALT` once at injection, exposes `hash(userId)` → 16-hex-char digest used by both `audit_events.actor_id_hash` (ADR-0013) and the future Pino `user_id_hash` (ADR-0012). Same salt + same input ⇒ same output ⇒ join key between the two streams.
- **`LOG_USER_ID_SALT` env var** promoted from the "future vars" block in `.env.example` to the active section, with the same boot-time validator pattern as `SESSION_SECRET` / `SESSION_ENCRYPTION_KEY`: mandatory, base64url, ≥ 32 bytes decoded, placeholder rejected. Wired in `main.ts`.
- **`AuditModule` is now `@Global()`** and also provides `HashUserIdService`. The previous in-line comment said "imported globally by AppModule" but the decorator was missing — without it, AuthController and the absolute-timeout middleware couldn't inject `AuditWriter` without re-importing AuditModule.
- **Emission points**:
  - `/auth/callback` happy path → `auth.sign_in` after `session.save()` (blocking per ADR-0013 §"Blocking writes": a failed audit fails the sign-in).
  - `/auth/callback` failure paths → `auth.sign_in.failed` with a discriminator `failureKind` (`entra-error`, `missing-code-or-state`, `no-pre-auth-cookie`, or any of the `AuthCodeFlowError` kinds — `state-mismatch`, `flow-expired`, `token-exchange-failed`).
  - `/auth/logout` (authenticated only) → `auth.sign_out` before `session.destroy()` — once destroy runs we lose the actor id.
  - Absolute-timeout middleware → `auth.session.expired` with `reason: 'absolute'` and `ageMs` for forensic granularity.

### Out of scope (next PRs)

- The other four v1 events from ADR-0013's catalogue (`auth.session.revoked`, `auth.token.validation.failed`, `auth.mfa.assertion.failed`, `authz.deny`) — no triggering code path exists today. They land with the admin "logout everywhere" route, downstream API access (ADR-0014), and the eventual `@RequireMfa()` / `@RequireAdmin` guards.
- Idle-timeout expiry is intentionally silent — Redis lets the key disappear with no BFF observation point. Per ADR-0010.
- Separate `AUDIT_DATABASE_URL` connection pool with `audit_writer`-only credentials — ADR-0013 marks it as the production hardening step, deferred behind `SET LOCAL ROLE` in v1.
- Retention purge job + startup self-test probe — deferred to the on-prem infrastructure ADR per ADR-0013.

### Notable choices

- **No CLS-populating middleware.** ADR-0013 anticipates an interceptor that puts `actorIdHash` on the request CLS so `AuditWriter.recordEvent` can pick it up automatically. For the four call sites in this PR, every emission path already has the user object in hand, so we pass `actorIdHash` explicitly via the typed methods and skip the middleware. It can land later when more routes need it.
- **Blocking on the happy path = strict ADR posture.** `audit.signIn` is awaited before the 302; a Postgres outage makes the sign-in fail (5xx) rather than silently producing an un-audited session. That's "no audit ⇒ no action" applied to authentication itself. Matches ADR-0013 §"Blocking writes" verbatim.
- **`signInFailed` skips the actor hash by default.** Most failure paths reject before any claim is parsed (state mismatch, expired flow). The interface accepts an optional `actor` for the rare identity-after-rejection case (future MFA assertion failure, etc.).

### Test plan

- [x] `pnpm nx test portal-bff` (clean env) → **142/142 pass** (was 123; +19 new specs across `check-log-user-id-salt`, `hash-user-id.service`, `audit.service` typed-methods, `auth.controller`, `absolute-timeout.middleware`).
- [x] `pnpm nx lint portal-bff` → clean.
- [x] `pnpm nx build portal-bff` → clean.
- [x] **CI clean-env repro** (lesson from #115/#116/#117): every env var unset → tests still 142/142. The two module specs that previously sat on the boundary (`auth.module`, `session.module`) now bootstrap their own `@Global()` stub providers for `PrismaService` + `ClsService` so AuditWriter's transitive resolution works without booting Prisma for real.
- [ ] Manual smoke against running BFF + Postgres:
  - [ ] Sign in → `select * from audit.events where event_type = 'auth.sign_in'` returns one row with `actor_id_hash`, `subject = 'session:…'`, `payload.amr` populated.
  - [ ] Sign out → matching `auth.sign_out` row.
  - [ ] Force `SESSION_ABSOLUTE_TIMEOUT_SECONDS=5` + wait → `auth.session.expired` row with `payload.reason = 'absolute'` and `ageMs > 5000`.
  - [ ] Manual `UPDATE audit.events SET event_type = 'x' WHERE id = ...` as the BFF role → fails with "permission denied" (the role contract holds even when the migrator runs as a privileged login).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #120

apps/portal-bff/src/auth/auth.module.spec.ts

@@ -1,18 +1,40 @@
 import { randomBytes } from 'node:crypto';
 import { ConfidentialClientApplication } from '@azure/msal-node';
+import { Global, Module } from '@nestjs/common';
 import { Test } from '@nestjs/testing';
+import { ClsService } from 'nestjs-cls';
 import { LoggerModule } from 'nestjs-pino';
+import { PrismaService } from 'nestjs-prisma';
+import { AuditModule } from '../audit/audit.module';
 import { AuthModule } from './auth.module';
 import { ENTRA_CONFIG, type EntraConfig } from './entra-config.token';
 import { MSAL_CLIENT } from './msal-client.token';
 
-// AuthModule imports SessionModule (so AuthController can inject
-// `UserSessionIndexService`), which transitively pulls in
-// RedisModule + the session-secret / session-encryption-key
-// validators. The spec has to satisfy all of those env vars or the
-// test fails as soon as `compile()` walks the import graph — which
-// is exactly what bit the CI on PR #115 (CI starts with a clean env;
-// locally `nx test` was loading apps/portal-bff/.env and masking it).
+// Production app makes PrismaService + ClsService globally available
+// via `PrismaModule.forRoot({ isGlobal: true })` and ObservabilityModule.
+// In a slice-of-graph spec we stub both via a tiny @Global() module
+// so AuditModule's transitive resolution of AuditWriter succeeds
+// without booting Prisma or nestjs-cls for real.
+@Global()
+@Module({
+  providers: [
+    { provide: PrismaService, useValue: {} },
+    { provide: ClsService, useValue: { get: () => undefined } },
+  ],
+  exports: [PrismaService, ClsService],
+})
+class TestStubsModule {}
+
+// AuthModule imports SessionModule + (transitively, via AuditModule
+// being @Global) needs the AuditWriter providers. compile() walks
+// the full import graph so the spec has to satisfy every validator
+// at boot time, including:
+//   - SessionModule's RedisModule (REDIS_URL)
+//   - SessionModule's middleware factory (SESSION_SECRET +
+//     SESSION_ENCRYPTION_KEY)
+//   - AuditModule's HashUserIdService (LOG_USER_ID_SALT)
+// Lesson from PR #115/#116: CI starts with a clean env, masking the
+// failure that nx-loaded apps/portal-bff/.env hides locally.
 const VALID = {
   ENTRA_INSTANCE_URL: 'https://login.microsoftonline.com/',
   ENTRA_TENANT_ID: 'aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee',
@@ -25,16 +47,26 @@ const VALID = {
   REDIS_URL: 'redis://default:test-pass@127.0.0.1:65535/0',
   SESSION_SECRET: randomBytes(32).toString('base64url'),
   SESSION_ENCRYPTION_KEY: randomBytes(32).toString('base64url'),
+  LOG_USER_ID_SALT: randomBytes(32).toString('base64url'),
+  // PrismaModule reads this at boot; an unreachable URL is fine in
+  // tests because we never issue queries.
+  DATABASE_URL: 'postgresql://test:test@127.0.0.1:65535/test',
 };
 
 // AuthModule's MSAL_CLIENT factory injects the Pino-backed `Logger`
 // (which the production app supplies through `ObservabilityModule`).
-// Importing `LoggerModule.forRoot({ pinoHttp: { level: 'silent' } })`
-// here registers the same provider for the testing module without
-// flooding test stdout.
+// `AuditModule` is `@Global()` in production via AppModule; here we
+// import it explicitly because we compile a slice of the graph.
+// AuditWriter's deps (PrismaService, ClsService) are stubbed — the
+// spec doesn't need a working DB, only that the wiring resolves.
 async function compile() {
   return Test.createTestingModule({
-    imports: [LoggerModule.forRoot({ pinoHttp: { level: 'silent' } }), AuthModule],
+    imports: [
+      LoggerModule.forRoot({ pinoHttp: { level: 'silent' } }),
+      TestStubsModule,
+      AuditModule,
+      AuthModule,
+    ],
   }).compile();
 }