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.controller.ts

@@ -1,6 +1,7 @@
 import { Controller, Get, Inject, Query, Req, Res } from '@nestjs/common';
 import type { Request, Response } from 'express';
 import { Logger } from 'nestjs-pino';
+import { AuditWriter } from '../audit/audit.service';
 import { readSessionTimeouts, sessionCookieName } from '../session/session-cookie';
 import { UserSessionIndexService } from '../session/user-session-index.service';
 import {
@@ -29,6 +30,7 @@ export class AuthController {
     private readonly logger: Logger,
     @Inject(ENTRA_CONFIG) private readonly entra: EntraConfig,
     private readonly userSessionIndex: UserSessionIndexService,
+    private readonly audit: AuditWriter,
   ) {}
 
   /**
@@ -90,10 +92,15 @@ export class AuthController {
         },
         'AuthCallback',
       );
+      await this.audit.signInFailed({
+        failureKind: 'entra-error',
+        payload: { entraError, entraErrorDescription },
+      });
       return this.redirectWithError(res, 'token-exchange-failed');
     }
 
     if (typeof code !== 'string' || typeof state !== 'string') {
+      await this.audit.signInFailed({ failureKind: 'missing-code-or-state' });
       return this.redirectWithError(res, 'token-exchange-failed');
     }
 
@@ -103,6 +110,7 @@ export class AuthController {
       // their browser dropped the cookie (TTL elapsed in
       // a 3rd-party-cookie blocker, etc.). Treat as flow-expired.
       this.logger.warn({ event: 'auth.no_pre_auth_cookie' }, 'AuthCallback');
+      await this.audit.signInFailed({ failureKind: 'no-pre-auth-cookie' });
       return this.redirectWithError(res, 'flow-expired');
     }
 
@@ -127,6 +135,12 @@ export class AuthController {
       // and revoke. Best-effort: a Redis hiccup here doesn't fail
       // sign-in (the service swallows + logs).
       await this.userSessionIndex.add(user.oid, req.sessionID);
+      // First write to the audit trail — blocking, per ADR-0013.
+      // If this throws the user does NOT see a successful sign-in:
+      // the exception propagates and the controller emits a 5xx via
+      // Nest's default exception filter. Same posture as the
+      // session.save() above.
+      await this.audit.signIn({ actor: user, sessionId: req.sessionID });
       this.logger.log(
         {
           event: 'auth.signed_in',
@@ -141,6 +155,7 @@ export class AuthController {
     } catch (err) {
       if (err instanceof AuthCodeFlowException) {
         this.logger.warn({ event: 'auth.flow_error', failure: err.failure }, 'AuthCallback');
+        await this.audit.signInFailed({ failureKind: err.failure.kind });
         return this.redirectWithError(res, err.failure.kind);
       }
       throw err;
@@ -192,6 +207,12 @@ export class AuthController {
     // requests (nothing was ever added).
     if (user) {
       await this.userSessionIndex.remove(user.oid, sessionId);
+      // Audit the sign-out before tearing the session down — once
+      // destroy() runs we lose the actor id. Blocking per ADR-0013:
+      // if the audit row can't be written, the user does NOT get a
+      // "you're logged out" experience, because we can't certify
+      // the sign-out happened.
+      await this.audit.signOut({ actor: user, sessionId });
     }
 
     try {