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