feat(portal-bff): absolute-timeout middleware + user_sessions index per ADR-0010 (#115)

## Summary

Hardens the BFF session per ADR-0010 §"TTL policy" and §"Revocation":

- **Absolute-timeout middleware** — every request that survives `express-session` runs through a new middleware that checks `req.session.absoluteExpiresAt`. Past the 12 h hard ceiling, the middleware destroys the Redis-side session, clears the `portal_session` cookie, drops the entry from the per-user index, and lets the request continue anonymously. Route-level guards (`/me`, future `@RequireAuth`) turn that into a 401 where the user actually needs auth — public routes keep serving.
- **`user_sessions:{userId}` secondary index** — a new `UserSessionIndexService` maintains a Redis set of active session ids per user. Hooked into `/auth/callback` (SADD on sign-in) and `/auth/logout` + the absolute-timeout middleware (SREM on destroy). Best-effort: a failed `SADD`/`SREM` logs a warning and the auth flow continues. No in-product consumer in this PR — the admin "logout everywhere" endpoint lands with the admin module.
- **Session payload extension** — `createdAt` and `absoluteExpiresAt` are now set on the session at the same moment as `req.session.user` (in `/auth/callback`). The `session.types.ts` declaration merging exposes them as optional `SessionData` fields.

## Notable choices

**Non-intrusive enforcement on expiry.** ADR-0010 says "returns 401"; we interpret that as "the user eventually sees a 401 when they touch something that needs auth", not "every route returns 401 the moment we notice the ceiling". The middleware destroys the session and calls `next()` — `/me` returns 401 on its own (no user on the session), public routes stay accessible. Validated with the project lead 2026-05-12.

**Express middleware exposed via DI, not a NestJS `MiddlewareConsumer`.** Same pattern as `SESSION_MIDDLEWARE`: factory inside `SessionModule`, resolved from the application context in `main.ts` with `app.get<RequestHandler>(SESSION_ABSOLUTE_TIMEOUT_MIDDLEWARE)`. Keeps the wiring co-located with the session middleware and avoids the `AppModule.configure(consumer)` boilerplate for a one-off enforcement layer.

**Best-effort index maintenance.** `UserSessionIndexService.add` / `remove` catch Redis errors and log a Pino warning instead of throwing. Rationale (per ADR-0010): the index is a convenience for admin operations, not a security invariant — a Redis hiccup must not break sign-in / sign-out. Orphans (entries pointing to keys that have expired idle-TTL on their own) are tolerated and will be filtered by future consumer code.

**Per-user index identifier = Entra `oid`.** Stable per-user inside the tenant, matches `req.session.user.oid`. Admin "logout user X" will work against this same key. Future multi-tenant scenarios may want `${tid}:${oid}` — easy refactor when External ID activation lands (ADR-0008).

## Out of scope (next PRs)

- Admin "logout everywhere" endpoint consuming `UserSessionIndexService.list(userId)`. Waits on the admin module + `@RequireAdmin` / `@RequireMfa` guards.
- Audit-pipeline first-class events for `session.absolute_timeout` and `user_session_index.*` (ADR-0013). For now they're structured Pino logs.
- Token blob persistence (id_token / access_token / refresh_token) in the encrypted session — ADR-0014 dependency.

## Test plan

- [x] `pnpm nx test portal-bff` → **123/123 pass** (was 110 before; +13 specs across new `user-session-index.service.spec.ts`, `absolute-timeout.middleware.spec.ts`, and added cases in `auth.controller.spec.ts`).
- [x] `pnpm nx lint portal-bff` → clean.
- [x] `pnpm nx build portal-bff` → clean webpack build.
- [x] Prettier-clean for all touched files.
- [ ] Manual smoke against running BFF:
  - [ ] Sign in normally → Redis has `session:<id>` + `user_sessions:<oid>` SISMEMBER returns `<id>`.
  - [ ] Logout → both keys gone.
  - [ ] Forge a past `absoluteExpiresAt` in Redis (or shorten `SESSION_ABSOLUTE_TIMEOUT_SECONDS=5` in `.env`) → next request after expiry returns 401 on `/me`, cookie cleared, index entry SREM-ed.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #115

apps/portal-bff/src/auth/auth.controller.ts

@@ -1,7 +1,8 @@
 import { Controller, Get, Inject, Query, Req, Res } from '@nestjs/common';
 import type { Request, Response } from 'express';
 import { Logger } from 'nestjs-pino';
-import { sessionCookieName } from '../session/session-cookie';
+import { readSessionTimeouts, sessionCookieName } from '../session/session-cookie';
+import { UserSessionIndexService } from '../session/user-session-index.service';
 import {
   PRE_AUTH_COOKIE_NAME,
   clearPreAuthCookieOptions,
@@ -27,6 +28,7 @@ export class AuthController {
     private readonly authService: AuthService,
     private readonly logger: Logger,
     @Inject(ENTRA_CONFIG) private readonly entra: EntraConfig,
+    private readonly userSessionIndex: UserSessionIndexService,
   ) {}
 
   /**
@@ -106,13 +108,25 @@ export class AuthController {
 
     try {
       const user = await this.authService.completeAuthCodeFlow(code, state, preAuth);
+      const now = Date.now();
+      const { absoluteSeconds } = readSessionTimeouts();
       req.session.user = user;
+      req.session.createdAt = now;
+      // Hard ceiling per ADR-0010 §"TTL policy" — checked on every
+      // request by the absolute-timeout middleware, independent of
+      // idle TTL.
+      req.session.absoluteExpiresAt = now + absoluteSeconds * 1000;
       // Force the save before the redirect: express-session writes
       // on response end, but the 302 we're about to emit closes the
       // response before the async store-write would otherwise
       // complete. Without this, the browser hits the SPA before
       // Redis carries the new payload.
       await saveSession(req);
+      // Register the freshly-minted session id in the per-user
+      // index so a future admin "logout everywhere" can enumerate
+      // 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);
       this.logger.log(
         {
           event: 'auth.signed_in',
@@ -168,9 +182,18 @@ export class AuthController {
    */
   @Get('logout')
   async logout(@Req() req: Request, @Res() res: Response): Promise<void> {
-    const wasAuthenticated = Boolean(req.session.user);
+    const user = req.session.user;
+    const wasAuthenticated = Boolean(user);
+    const sessionId = req.sessionID;
     const logoutUrl = this.authService.buildLogoutUrl();
 
+    // Drop the user_sessions index entry before destroy() removes
+    // the session id from `req`. Skipped on already-anonymous
+    // requests (nothing was ever added).
+    if (user) {
+      await this.userSessionIndex.remove(user.oid, sessionId);
+    }
+
     try {
       await destroySession(req);
     } catch (err) {