feat(portal-bff): extract Entra roles claim onto AuthenticatedUser (#126)

## Summary

First step in the `portal-admin` audit-log-viewer workstream (per [ADR-0020](docs/decisions/0020-portal-admin-app.md)). The BFF's `AdminRoleGuard` (next PR) needs to read `session.user.roles` to enforce admin-only access to `/api/admin/*`. Today the session carries `{ oid, tid, username, displayName, amr }` — the `roles` claim is dropped on the floor when the ID token comes back from Entra.

This PR closes that gap:

- Adds `roles: readonly string[]` to [AuthenticatedUser](apps/portal-bff/src/auth/auth.service.ts) and threads it through `toAuthenticatedUser()`.
- The field flows onto `req.session.user` automatically via the existing module-augmentation chain in [session.types.ts](apps/portal-bff/src/session/session.types.ts) — no extra wiring.

## Defensive parsing

Mirrors the existing `amr` extraction pattern:

| Input claim shape | Result |
| --- | --- |
| `["admin", "editor"]` | `["admin", "editor"]` |
| Claim absent | `[]` |
| Non-array (e.g. `"admin"`) | `[]` |
| Mixed types (e.g. `["admin", 42, null, "editor"]`) | `["admin", "editor"]` |

Empty array means **"user has no app role assigned"**, not **"claim was unparseable"** — both collapse to the same value because both are equally non-authoritative for the admin guard.

## Why this is its own PR

The `AdminRoleGuard` + `@RequireAdmin()` decorator + first `/api/admin/me` self-test endpoint will follow in the next PR. Splitting the claim extraction out makes both diffs trivial to read and lets the second PR focus on guard semantics + audit emission without the mechanical fixture updates that came with adding a new `AuthenticatedUser` field.

## Surface impact — none yet

- `PublicUser` (the SPA-facing shape returned by `GET /api/auth/me`) is **deliberately unchanged**. Exposing `roles` to the SPA happens in the next PR alongside the conditional admin-link rendering — without a consumer in this PR it would be dead code.
- Audit pipeline unchanged. `SignInActor` carries `{ oid, amr }` only; the audit log doesn't need `roles` and won't get it.
- No new env vars, no new dependencies.

## Test plan

- [x] `pnpm nx test portal-bff` — **203 specs pass** (was 199; +4 new specs covering the four parsing cases above).
- [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] Existing fixtures in [auth.controller.spec.ts](apps/portal-bff/src/auth/auth.controller.spec.ts), [auth.service.spec.ts](apps/portal-bff/src/auth/auth.service.spec.ts), [absolute-timeout.middleware.spec.ts](apps/portal-bff/src/session/absolute-timeout.middleware.spec.ts) updated with `roles: []`.
- [ ] e2e — would require the `admin` app role to be declared on the Entra registration and assigned to a test user. Out of scope for this PR; will be validated when the `AdminRoleGuard` lands and there is a 403 to observe.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #126

apps/portal-bff/src/auth/auth.service.spec.ts

@@ -35,20 +35,29 @@ function makeService(overrides?: { acquireTokenByCode?: jest.Mock }): ServiceFix
 function makeAuthResult(
   claims: Partial<{
     amr: string[];
+    roles: unknown;
     oid: string;
     tid: string;
     name: string;
     preferred_username: string;
   }>,
 ): AuthenticationResult {
+  const idTokenClaims: Record<string, unknown> = {
+    oid: claims.oid ?? 'user-oid',
+    tid: claims.tid ?? ENTRA.tenantId,
+    name: claims.name ?? 'Jane Doe',
+    preferred_username: claims.preferred_username ?? 'jane.doe@apf.example',
+    amr: claims.amr ?? ['pwd', 'mfa'],
+  };
+  // `roles` is optional in Entra ID tokens — only present when the
+  // user has at least one app role assigned. The fixture omits it
+  // unless the caller explicitly opts in (including with falsy
+  // values to exercise the empty-array fallback).
+  if (Object.prototype.hasOwnProperty.call(claims, 'roles')) {
+    idTokenClaims['roles'] = claims.roles;
+  }
   return {
-    idTokenClaims: {
-      oid: claims.oid ?? 'user-oid',
-      tid: claims.tid ?? ENTRA.tenantId,
-      name: claims.name ?? 'Jane Doe',
-      preferred_username: claims.preferred_username ?? 'jane.doe@apf.example',
-      amr: claims.amr ?? ['pwd', 'mfa'],
-    },
+    idTokenClaims,
     account: { username: 'jane.doe@apf.example', name: 'Jane Doe' },
   } as unknown as AuthenticationResult;
 }
@@ -111,6 +120,7 @@ describe('AuthService.completeAuthCodeFlow', () => {
       username: 'jane.doe@apf.example',
       displayName: 'Jane Doe',
       amr: ['pwd', 'mfa'],
+      roles: [],
     });
   });
 
@@ -144,6 +154,58 @@ describe('AuthService.completeAuthCodeFlow', () => {
     expect(user.oid).toBe('user-oid');
   });
 
+  it('surfaces the `roles` claim when Entra includes it (app-role-assigned user)', async () => {
+    const acquireTokenByCode = jest
+      .fn()
+      .mockResolvedValue(makeAuthResult({ roles: ['admin', 'editor'] }));
+    const { service } = makeService({ acquireTokenByCode });
+    const user = await service.completeAuthCodeFlow(
+      'code',
+      PRE_AUTH_OK.state,
+      PRE_AUTH_OK,
+      PRE_AUTH_OK.createdAt,
+    );
+    expect(user.roles).toEqual(['admin', 'editor']);
+  });
+
+  it('returns an empty `roles` array when the claim is absent (no app role assigned)', async () => {
+    const acquireTokenByCode = jest.fn().mockResolvedValue(makeAuthResult({}));
+    const { service } = makeService({ acquireTokenByCode });
+    const user = await service.completeAuthCodeFlow(
+      'code',
+      PRE_AUTH_OK.state,
+      PRE_AUTH_OK,
+      PRE_AUTH_OK.createdAt,
+    );
+    expect(user.roles).toEqual([]);
+  });
+
+  it('returns an empty `roles` array when the claim is non-array (defensive)', async () => {
+    const acquireTokenByCode = jest.fn().mockResolvedValue(makeAuthResult({ roles: 'admin' }));
+    const { service } = makeService({ acquireTokenByCode });
+    const user = await service.completeAuthCodeFlow(
+      'code',
+      PRE_AUTH_OK.state,
+      PRE_AUTH_OK,
+      PRE_AUTH_OK.createdAt,
+    );
+    expect(user.roles).toEqual([]);
+  });
+
+  it('drops non-string entries from `roles` (defensive against unexpected claim shapes)', async () => {
+    const acquireTokenByCode = jest
+      .fn()
+      .mockResolvedValue(makeAuthResult({ roles: ['admin', 42, null, 'editor'] }));
+    const { service } = makeService({ acquireTokenByCode });
+    const user = await service.completeAuthCodeFlow(
+      'code',
+      PRE_AUTH_OK.state,
+      PRE_AUTH_OK,
+      PRE_AUTH_OK.createdAt,
+    );
+    expect(user.roles).toEqual(['admin', 'editor']);
+  });
+
   it('throws token-exchange-failed when MSAL throws', async () => {
     const acquireTokenByCode = jest.fn().mockRejectedValue(new Error('AADSTS70008'));
     const { service } = makeService({ acquireTokenByCode });