feat(portal-bff): admin audit-log read endpoint + audit_reader-locked path (#132)

## Summary

First real consumer of the admin module — `GET /api/admin/audit`, the paginated audit-log viewer named in [ADR-0020](docs/decisions/0020-portal-admin-app.md)'s v1 catalogue. Gated by `@RequireAdmin`, reads through the `audit_reader` Postgres role only, and emits `admin.audit.query` on every call as the "fishing expedition" deterrent ADR-0020 calls out (§"Read actions are also captured … to deter fishing expeditions"). This is the BFF half of the audit-viewer chantier — the SPA screen lands later.

## What ships

### [`AuditReader`](apps/portal-bff/src/admin/audit-reader.service.ts)

- Wraps every read in a transaction whose first statement is `SET LOCAL ROLE audit_reader`. Symmetric with `AuditWriter`'s `SET LOCAL ROLE audit_writer`: the runtime role contract holds even when the BFF connection is otherwise privileged. UPDATE / INSERT / DELETE on `audit.events` fail at the Postgres level regardless of what gets through the application layer.
- Parameterised SELECT only — filter values flow into `$queryRawUnsafe`'s positional params, never concatenated into the SQL string. Subject-prefix filter uses `LIKE` with explicit `ESCAPE '\\'` and escapes `%` / `_` / `\` in the literal so an admin-side wildcard can't masquerade as a meta-character.
- COUNT(\*) + `LIMIT` / `OFFSET` pagination. Ordering is `created_at DESC, id DESC` for deterministic page boundaries on identical timestamps (UUIDs break ties).
- Hard caps `limit` at `MAX_LIMIT` (200) even if a caller bypasses the DTO — defense in depth on the BFF's event loop.

### [`AdminAuditQueryDto`](apps/portal-bff/src/admin/audit-query.dto.ts)

| Filter | Type | Notes |
| --- | --- | --- |
| `eventType` | string ≤128 | Exact match (e.g. `auth.sign_in`). |
| `actorIdHash` | string ≤128 | Exact match on the salted hash from the writer. |
| `audience` | enum | `workforce` \| `customer`. |
| `outcome` | enum | `success` \| `failure` \| `denied`. |
| `subjectPrefix` | string ≤128 | `LIKE 'prefix%'`, escaped literal. |
| `createdAtFrom` | ISO-8601 | Inclusive lower bound. |
| `createdAtTo` | ISO-8601 | Exclusive upper bound. |
| `limit` | int 1..200 | Default **50**. |
| `offset` | int ≥0 | Default **0**. |

Bound through Nest's global `ValidationPipe` — unknown query keys are rejected by `forbidNonWhitelisted` (defends against query-string smuggling), `transform: true` coerces numeric strings into numbers.

### [`AdminAuditController`](apps/portal-bff/src/admin/admin-audit.controller.ts)

`@Controller('admin/audit')` + `@RequireAdmin()` at the class level. The handler:

1. Calls `AuditReader.findEvents(filters)`.
2. Emits `admin.audit.query` with `{ filters, resultCount }` so a reviewer can see exactly what the admin searched for and how many rows came back.
3. Returns the page to the SPA.

`@RequireMfa({ freshness: 600 })` is **intentionally not applied** in v1: the admin surface already sits behind a freshly-MFA'd session at sign-in (per ADR-0020), and the per-query audit row is the deterrent. Adding `@RequireMfa` later is a one-line change — that's why the decorator was designed-in by PR #128.

### [`AuditWriter.adminAuditQuery()`](apps/portal-bff/src/audit/audit.service.ts)

New typed method using `outcome=success`. The read **happened** regardless of whether it matched rows; row count lives in `resultCount`. An `outcome=denied` from this surface is reserved for the day we add per-row authZ.

## Operational notes

- **Dev pool, `SET LOCAL ROLE` pattern** (per [ADR-0018](docs/decisions/0018-environment-configuration-strategy.md)): the BFF talks to Postgres on the shared `DATABASE_URL` pool and switches role per-transaction. In production the audit-write pool is already split via `AUDIT_DATABASE_URL`; a dedicated `audit_reader`-only pool is a future follow-up if read-side isolation is desired (the role-locking on the shared pool already prevents privilege bleed at the Postgres level).
- COUNT(\*) is fine at v1 audit volume; if the table grows past a few million rows we'll switch to keyset pagination and drop the total.

## Test plan

- [x] `pnpm nx test portal-bff` — **308 specs pass** (was 278; +30: AuditReader 10, AdminAuditController 6, AuditWriter typed event 2, DTO validation 12).
- [x] `pnpm exec nx affected -t format:check lint test build --base=origin/main` — clean (the pre-existing `_res` / `_next` warnings in `rate-limit.middleware.ts` are unrelated).
- [x] SQL-injection probe via fixture (`'; DROP TABLE events; --` as `eventType`) — value lands in the params array, SQL stays templated.
- [x] LIKE escaping verified: `%`, `_`, `\` in `subjectPrefix` are escaped to their literal form.
- [ ] e2e — pending the admin SPA + at least one `admin` Entra role assignment. Once both exist: `curl --cookie 'portal_admin_session=…' /api/admin/audit?eventType=auth.sign_in&limit=10` returns the most recent sign-ins and `psql` shows the matching `admin.audit.query` row in `audit.events`.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #132

apps/portal-bff/src/admin/admin.module.ts

@@ -1,8 +1,10 @@
 import { Module } from '@nestjs/common';
 import { AuthModule } from '../auth/auth.module';
+import { AdminAuditController } from './admin-audit.controller';
 import { AdminAuthController } from './admin-auth.controller';
 import { AdminController } from './admin.controller';
 import { AdminRoleGuard } from './admin-role.guard';
+import { AuditReader } from './audit-reader.service';
 
 /**
  * `AdminModule` — root of the `/api/admin/*` surface per ADR-0020.
@@ -28,7 +30,7 @@ import { AdminRoleGuard } from './admin-role.guard';
  */
 @Module({
   imports: [AuthModule],
-  controllers: [AdminController, AdminAuthController],
-  providers: [AdminRoleGuard],
+  controllers: [AdminController, AdminAuthController, AdminAuditController],
+  providers: [AdminRoleGuard, AuditReader],
 })
 export class AdminModule {}