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

apps/portal-bff/prisma/schema.prisma

@@ -1,13 +1,84 @@
-// 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`
+// Prisma schema for portal-bff.
+//
+// `multiSchema` preview is enabled because per ADR-0013 the audit log
+// lives in its own `audit` schema with role-based append-only access
+// (audit_owner / audit_writer / audit_reader / audit_archiver). The
+// public schema holds the regular business data; only audit.events
+// lives in audit.
 
 generator client {
-  provider = "prisma-client-js"
+  provider        = "prisma-client-js"
+  previewFeatures = ["multiSchema"]
 }
 
 datasource db {
   provider = "postgresql"
   url      = env("DATABASE_URL")
+  schemas  = ["public", "audit"]
+}
+
+// ============================================================
+// Audit log (per ADR-0013)
+// ============================================================
+//
+// Append-only by Postgres role grants — the schema, roles, and
+// default privileges are provisioned by infra/local/init/postgres/
+// 01-init.sql in dev and the equivalent production manifest. The
+// migration that creates this table re-applies the grants
+// explicitly and ALTERs the table owner to `audit_owner` so the
+// runtime role contract holds even when the migration runs as a
+// privileged migrator account.
+//
+// At runtime, the BFF wraps every INSERT into this table in a
+// transaction that begins with `SET LOCAL ROLE audit_writer`, so
+// even a compromised BFF connection cannot UPDATE / TRUNCATE /
+// DELETE — those grants are not on `audit_writer`.
+
+enum AuditAudience {
+  workforce
+  customer
+
+  @@schema("audit")
+}
+
+enum AuditOutcome {
+  success
+  failure
+  denied
+
+  @@schema("audit")
+}
+
+model AuditEvent {
+  id          String        @id @default(uuid()) @db.Uuid
+  createdAt   DateTime      @default(now()) @map("created_at") @db.Timestamptz(6)
+  eventType   String        @map("event_type")
+  audience    AuditAudience
+  // Salted hash (LOG_USER_ID_SALT) of the actor's stable id. NULL
+  // when the actor is unauthenticated (e.g. failed login attempt
+  // before resolving an identity). The same salt is used by the
+  // BFF Pino logger so audit and app logs cross-correlate on this
+  // field.
+  actorIdHash String?       @map("actor_id_hash")
+  // W3C trace id (32 hex chars) of the request that produced the
+  // event. Cross-correlates with traces in Jaeger and with Pino
+  // log lines that carry the same `trace_id` field. NULL only if
+  // the event was emitted outside any inbound request (e.g. a
+  // future cron job).
+  traceId     String?       @map("trace_id")
+  // Free-form identifier of what the event is *about* — typically
+  // a domain entity URI like `user:42` or `dossier:xyz`. NULL when
+  // the event is system-wide and has no clear subject.
+  subject     String?
+  outcome     AuditOutcome
+  // Event-specific structured detail. Redaction of PII is the
+  // caller's responsibility; the BFF Pino redact list is the
+  // reference allow-/deny-list.
+  payload     Json?
+
+  @@map("events")
+  @@schema("audit")
+  @@index([createdAt])
+  @@index([eventType])
+  @@index([traceId])
 }