feat(portal-bff): audit log foundation per ADR-0013 (#76)

## Summary

Lays down the append-only audit log per ADR-0013: schema declaration, first migration with role grants, NestJS `AuditWriter` service. Typed event-family methods, the separate `AUDIT_DATABASE_URL` pool, the retention job, and the live-DB integration tests are explicitly listed as "wired as features land" in the ADR's confirmation block — they ship when the matching feature ADRs do.

## What lands

**Prisma schema** ([`apps/portal-bff/prisma/schema.prisma`](apps/portal-bff/prisma/schema.prisma)):

- `multiSchema` preview enabled; datasource declares `public` + `audit` schemas.
- `AuditEvent` model: `id` (uuid), `createdAt`, `eventType` (free-form in v1), `audience` enum (`workforce | customer`), `actorIdHash`, `traceId`, `subject`, `outcome` enum (`success | failure | denied`), `payload` (jsonb).
- Indexes on `createdAt`, `eventType`, `traceId` — covering the three obvious query shapes.

**Migration** ([`prisma/migrations/*_init_audit_schema/migration.sql`](apps/portal-bff/prisma/migrations/20260510011453_init_audit_schema/migration.sql)):

- Standard Prisma `CREATE TABLE` / enums output, then the **append-only contract** re-applied explicitly:
  - `ALTER TABLE/TYPE OWNER TO audit_owner`.
  - `GRANT INSERT` to `audit_writer`, `SELECT` to `audit_reader`, **`SELECT, DELETE`** to `audit_archiver` (SELECT is needed to evaluate the `created_at` predicate of "delete older than retention" — Postgres requires SELECT on every column referenced in DELETE's WHERE).
  - `GRANT USAGE` on the enum types to all three roles (without it `audit_writer.INSERT` fails with "permission denied for type").
  - **No** GRANT for `UPDATE` / `TRUNCATE` to anyone — including `audit_owner` at runtime; only fresh schema migrations amend the table.

**Service** ([`apps/portal-bff/src/audit/`](apps/portal-bff/src/audit/)):

- `AuditWriter.recordEvent(input)` — single entry point. Wraps every INSERT in a transaction whose first statement is `SET LOCAL ROLE audit_writer`, so the role contract holds at runtime even from the otherwise-privileged BFF connection.
- `traceId` auto-resolved from the active OTel span (so audit row joins with traces and Pino logs on the same `trace_id`).
- `actorIdHash` auto-resolved from CLS (key `actorIdHash`) with explicit input-side override; `null` when neither is set (placeholder until ADR-0009 / ADR-0010 guards populate CLS).
- Errors propagate (no catch-and-swallow), per ADR-0013's "blocking writes: no audit ⇒ no action".

**Tests** — 8 unit tests on `AuditWriter` (mocked Prisma + CLS): role-locking ordering, input pass-through, `Prisma.JsonNull` for missing payload, CLS-vs-input precedence on `actorIdHash`, OTel trace capture, error propagation.

## End-to-end verification (manual, against local-dev Postgres)

```
INSERT under audit_writer:   ok
UPDATE under audit_writer:   permission denied for table events
DELETE under audit_writer:   permission denied for table events
DELETE under audit_archiver: ok, row removed (after the SELECT-grant fix)
```

## ADR-0013 §Confirmation rewritten

Two-block split: "wired in foundation PR" lists what landed here; "wired as features land" lists the typed event-family methods, AUDIT_DATABASE_URL connection split, startup self-test probe, retention purge job, salt-shared cross-correlation test, and live-DB role-contract integration tests — each anchored to the feature ADR that triggers it.

## Recovery for anyone with a pre-existing local-dev DB

If your local-dev Postgres already had the audit migration applied **before** the SELECT-grant fix, the archiver's DELETE will fail. Two options:

1. Apply the missing grant directly:
   ```bash
   psql "$DATABASE_URL" -c "GRANT SELECT ON audit.events TO audit_archiver;"
   ```
2. Or wipe the volume and re-migrate cleanly:
   ```bash
   ./infra/local/dev.sh down -v
   ./infra/local/dev.sh up
   pnpm --filter @apf-portal/source exec prisma migrate deploy   # or `cd apps/portal-bff && pnpm exec prisma migrate deploy`
   ```

Fresh DBs land with the corrected migration directly.

## Out of scope (separate PRs)

- Typed event-family methods (`signIn`, `signInFailed`, …) — added per matching feature ADR.
- `AUDIT_DATABASE_URL` separate connection pool — defense-in-depth, when production needs it.
- Startup self-test probe (deliberate failing UPDATE asserting rejection) — lands with the connection split.
- Retention purge job (`audit_archiver` daily cron) — phase-3b infra.
- Live-DB integration tests asserting the role contract — Testcontainers-style harness, separate PR.

## Test plan

- [ ] CI green on this PR.
- [ ] `prisma migrate deploy` succeeds on a fresh DB (the recovery instructions cover the SELECT-grant gap for already-migrated dev DBs).
- [ ] `psql -c "\dp audit.events"` shows the expected privilege matrix: `audit_owner=arwdDxtm/audit_owner`, `audit_writer=a/audit_owner`, `audit_reader=r/audit_owner`, `audit_archiver=rd/audit_owner`.
- [ ] BFF boots; calling `AuditWriter.recordEvent` from a controller (manual smoke once a real flow lands) writes to `audit.events` with the expected `trace_id` matching the request's Jaeger span.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #76

docs/decisions/0013-audit-trail-separated-postgres-append-only.md

@@ -156,14 +156,25 @@ Hooks for **admin actions** and **sensitive data access** are designed-in: the w
 
 ### Confirmation
 
-- `apps/portal-bff/prisma/schema.prisma` enables `multiSchema` and declares the `audit` schema; the `AuditEvent` model and `AuditOutcome` enum live in it.
-- Three Postgres roles exist: `audit_writer` (INSERT only on `audit.events`), `audit_reader` (SELECT only), `audit_archiver` (DELETE only, on rows older than the configured retention). The schema migration includes the explicit `REVOKE UPDATE, DELETE, TRUNCATE FROM PUBLIC` on `audit.events` and the targeted grants.
-- `apps/portal-bff/src/audit/audit.module.ts` provides an `AuditService` with one typed method per event family (`signIn`, `signInFailed`, `signOut`, `sessionExpired`, `sessionRevoked`, `tokenValidationFailed`, `mfaAssertionFailed`, `authzDeny`, plus dormant `adminAction` and `sensitiveDataAccess`).
-- `AuditService` connects via `AUDIT_DATABASE_URL` as `audit_writer`; a startup probe asserts that the role can `INSERT` and cannot `UPDATE` (a deliberate failing UPDATE during boot is rejected, the BFF starts; if it succeeds, the BFF refuses to start).
-- The auth controller, token-validation interceptor, MFA guard, sessions module, and authorization guards each call `AuditService` on the relevant outcomes. Tests assert one audit row per event.
-- The retention purge runs daily; failure raises an alert and emits `audit.retention.purge` with `outcome = failure`.
-- The same `LOG_USER_ID_SALT` is used by app logs and by audit; an integration test asserts that the same user produces the same `actor_id_hash` in both streams.
-- `AUDIT_RETENTION_DAYS < 30` and missing required env vars prevent BFF startup.
+**Wired in the foundation PR:**
+
+- `apps/portal-bff/prisma/schema.prisma` enables the `multiSchema` preview, declares the `audit` schema alongside `public`, and carries the `AuditEvent` model with `AuditAudience` (`workforce | customer`) and `AuditOutcome` (`success | failure | denied`) enums.
+- The migration `prisma/migrations/*_init_audit_schema/migration.sql` creates `audit.events`, `ALTER`s table + enum types to be owned by `audit_owner`, and re-applies the role grants explicitly: `INSERT` to `audit_writer`, `SELECT` to `audit_reader`, `SELECT, DELETE` to `audit_archiver` (SELECT is needed for archiver to evaluate the `created_at` predicate of "delete older than retention"). No grant of `UPDATE` or `TRUNCATE` to anyone — including the migrator's own login at runtime; only fresh schema migrations amend the table.
+- The roles themselves and the schema with default privileges are provisioned earlier by `infra/local/init/postgres/01-init.sql` (dev) — production replicates the same SQL via the future on-prem infrastructure ADR.
+- `apps/portal-bff/src/audit/audit.service.ts` exposes a single `AuditWriter.recordEvent(input)` method. Every write runs in a transaction whose first statement is `SET LOCAL ROLE audit_writer`, so the runtime contract holds even if the BFF connection is otherwise privileged. `trace_id` is auto-resolved from the active OTel span; `actor_id_hash` is read from CLS or accepted as an explicit override (placeholder until ADR-0009 / ADR-0010 land their guards). Failures propagate — no catch-and-swallow, per "blocking writes: no audit ⇒ no action".
+- BFF connects via the shared `DATABASE_URL` (the role switch is per-transaction). A separate `AUDIT_DATABASE_URL` connection pool is the production hardening, deferred — see "wired as features land" below.
+- Smoke-tested end to end against the local-dev Postgres: `audit_writer` INSERTs successfully, fails on `UPDATE` and `DELETE`; `audit_archiver` SELECTs + DELETEs successfully.
+
+**Wired as the corresponding features land:**
+
+- One typed method per event family on `AuditWriter` — `signIn`, `signInFailed`, `signOut`, `sessionExpired`, `sessionRevoked`, `tokenValidationFailed`, `mfaAssertionFailed`, `authzDeny`, `adminAction`, `sensitiveDataAccess` — added as the matching feature ships (ADR-0009 / ADR-0010 / ADR-0011 / future authz). v1 keeps the surface narrow with the single `recordEvent` so events can be emitted today (e.g. by the auth flow once it lands) while the typed catalogue accretes on a real basis.
+- `AUDIT_DATABASE_URL` separate connection (a distinct pool with `audit_writer`-only login credentials) — defense-in-depth that locks down what an at-runtime SQL injection can do further. v1 mitigates with `SET LOCAL ROLE` at the cost of sharing the pool with public-schema reads/writes; v2 splits.
+- Startup self-test probe — a deliberate failing `UPDATE` against `audit.events` during boot, asserting it is rejected; if it succeeds, the BFF refuses to start. Lands with the connection split above.
+- Retention purge job invoking the `audit_archiver` role daily via cron, emitting `audit.retention.purge` with `outcome = failure` on error. Operational concern — phase-3b infra ADR.
+- Auth controller, MFA guard, sessions module, and authorization guards each call `AuditWriter` on the relevant outcomes. Tests assert one audit row per event.
+- Integration test verifying that the same user produces the same `actor_id_hash` in both Pino logs and audit rows (same `LOG_USER_ID_SALT`, same hashing path) — wired with the auth + LOG_USER_ID_SALT enforcement.
+- `AUDIT_RETENTION_DAYS < 30` and missing required env vars prevent BFF startup — wired with the retention job.
+- Live-DB integration tests asserting the role contract (UPDATE/TRUNCATE rejected at runtime) — Testcontainers harness, separate PR.
 
 ## Pros and Cons of the Options