fix(portal-bff): align admin entra role name with Portal.Admin (#145)

## Summary

The [`AdminRoleGuard`](apps/portal-bff/src/admin/admin-role.guard.ts) was matching on the literal `'admin'`, but the Entra app registration declares the admin app role with `value: "Portal.Admin"`. End result: an authenticated user with the role assigned in Entra still landed with `roles: []` in their session (claim simply not present in the id token), and every request to `/api/admin/audit` and `/api/admin/users` returned a **403**.

Caught manually in the portal-admin SPA: login succeeded, sidebar links to "Audit log" / "User list" returned 403. The [`/api/admin/auth/me`](apps/portal-bff/src/admin/admin-auth.controller.ts) self-test confirmed the missing claim was the cause.

## What lands

### Constant value — single source of truth

[`apps/portal-bff/src/admin/admin-role.guard.ts:18`](apps/portal-bff/src/admin/admin-role.guard.ts#L18):

```diff
-export const ADMIN_ROLE = 'admin';
+export const ADMIN_ROLE = 'Portal.Admin';
```

[`admin-role.guard.spec.ts`](apps/portal-bff/src/admin/admin-role.guard.spec.ts) already imports `ADMIN_ROLE` from the source rather than hardcoding the literal, so the guard contract spec rolls through unchanged. The fixtures elsewhere ([`auth.service.spec.ts`](apps/portal-bff/src/auth/auth.service.spec.ts), [`admin.controller.spec.ts`](apps/portal-bff/src/admin/admin.controller.spec.ts), [`admin-auth.controller.spec.ts`](apps/portal-bff/src/admin/admin-auth.controller.spec.ts), [`require-mfa.guard.spec.ts`](apps/portal-bff/src/auth/require-mfa.guard.spec.ts)) keep `roles: ['admin']` as fixture data — those tests exercise the extraction / serialization pipeline, which is role-value-agnostic; touching them would be incidental cleanup with no behaviour signal.

### Doc-comment refresh

Inline references to the role name updated so future readers don't grep `'admin'` and find a phantom value:

- [`admin-role.guard.ts`](apps/portal-bff/src/admin/admin-role.guard.ts) — class doc-block (3 mentions).
- [`admin.controller.ts`](apps/portal-bff/src/admin/admin.controller.ts) — class doc-block + inline guard-contract comment.
- [`audit.service.ts`](apps/portal-bff/src/audit/audit.service.ts) — `adminAccessDenied` doc-block (2 mentions).

### Documentation

- [`docs/decisions/0020-portal-admin-app.md`](docs/decisions/0020-portal-admin-app.md) — 5 references to the role across §"How is admin access enforced", §"Auth — same Entra ID …", and the Consequences §.
- [`docs/architecture.md`](docs/architecture.md) — note next to the C4 container diagram describing the admin entry gate.
- [`CLAUDE.md`](CLAUDE.md) — "Admin application" project rule.

## Notes for the reviewer

- **Why `Portal.Admin` rather than `admin`?** Operator's call on the Entra side. The `<Application>.<Role>` namespace is the conventional Entra App Role pattern when the directory may host roles for multiple applications, and `admin` alone is ambiguous in a directory shared across products.
- **Why no migration / backfill?** The role value lives only in two places: Entra app-registration manifest (operator-managed) and the BFF constant (this PR). Existing Redis sessions captured `roles: []` (claim absent) — they'll naturally pick up the correct value on next sign-in. No persisted data references the old value.
- **No ADR.** ADR-0020 §"How is admin access enforced" already commits to "Entra ID role claim + BFF guard"; the literal role string is an implementation detail the ADR happened to spell. Updated the ADR's prose to the new value to keep the doc honest, but the decision is unchanged.

## Test plan

- [x] `pnpm nx test portal-bff` — **396 specs pass**, unchanged from `main`. The `AdminRoleGuard` contract spec (covers 401-on-no-session, 403-on-missing-role + audit emission, pass-through-on-role-present) imports `ADMIN_ROLE` and re-exercises with the new value.
- [x] `pnpm exec nx affected -t format:check lint test build --base=origin/main` — clean.
- [ ] Manual verification — pending Entra-side App Role declaration with `value: "Portal.Admin"` + assignment to the test user. Once both exist: sign out + sign in on portal-admin, hit `/api/admin/auth/me` and confirm `roles: ["Portal.Admin"]`, then click "Audit log" + "User list" and confirm both render. An `admin.access_denied` row in `audit.events` is the negative-test signal (still emitted for any user without the role).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #145

CLAUDE.md

@@ -48,7 +48,7 @@ The structural, security, observability, and quality choices are recorded as ADR
 - **Performance budgets:** Core Web Vitals at Google "Good" thresholds (LCP ≤ 2.5 s, INP ≤ 200 ms, CLS ≤ 0.1, TBT ≤ 200 ms, TTFB ≤ 800 ms), Lighthouse Performance ≥ 90 on critical routes. **Lighthouse CI** (`@lhci/cli`) runs in CI with median-of-3 mitigation, blocking on threshold breach. Angular bundle `budgets` (`type: "error"`): initial ≤ 300 KB gzip, lazy chunks ≤ 100 KB gzip. BFF p95/p99 SLOs per endpoint family observed via OTel (advisory in CI, alerting in prod). Weekly scheduled Lighthouse run on prod env. **a11y wins over perf** when they conflict — see [ADR-0017](docs/decisions/0017-performance-budgets-lighthouse-ci.md).
 - **Environment configuration:** SPA per-environment values via Angular `environment.ts` + `fileReplacements` at build time (no runtime config-fetch). BFF reads `process.env` directly with small per-key boot-time validators (no `@nestjs/config` overhead at this scale). The audit log uses a separate `AUDIT_DATABASE_URL` connection pool in production (`audit_writer`-only login, defense in depth) and falls back to the shared pool + `SET LOCAL ROLE` in dev — see [ADR-0018](docs/decisions/0018-environment-configuration-strategy.md).
 - **Internationalisation:** `@angular/localize` in build-time mode, two locales (`fr` default served at `/`, `en`), source locale = English (project English-only rule). Path-based URLs always prefixed (`/fr/...`, `/en/...`); `/` smart-redirects via cookie → `Accept-Language` → `fr`. UI strings live in XLIFF (`messages.fr.xlf`); editorial / CMS content is BFF-served already localised (see admin app). Footer hosts the locale switcher; switching writes a `__Host-portal_locale` cookie and hard-refreshes — see [ADR-0019](docs/decisions/0019-internationalisation-angular-localize.md).
-- **Admin application (`portal-admin`):** dedicated Angular SPA alongside `portal-shell`, sharing the same `portal-bff` via `/api/admin/*` routes guarded by an Entra `admin` role + `@RequireMfa({ freshness: 600 })` at entry. Distinct origin / cookie / session from `portal-shell` (`__Host-portal_admin_session`). v1 modules: CMS for static pages (multilingual), menu management, user list (read-only), audit log viewer. Bundle budget relaxed to ≤ 500 KB gzip (vs 300 KB for `portal-shell`); same a11y + dark-mode baseline. Shared UI primitives (`Icon`, `LayoutStateService`, brand tokens) graduate to `libs/shared/*` as both apps need them — see [ADR-0020](docs/decisions/0020-portal-admin-app.md).
+- **Admin application (`portal-admin`):** dedicated Angular SPA alongside `portal-shell`, sharing the same `portal-bff` via `/api/admin/*` routes guarded by an Entra `Portal.Admin` role + `@RequireMfa({ freshness: 600 })` at entry. Distinct origin / cookie / session from `portal-shell` (`__Host-portal_admin_session`). v1 modules: CMS for static pages (multilingual), menu management, user list (read-only), audit log viewer. Bundle budget relaxed to ≤ 500 KB gzip (vs 300 KB for `portal-shell`); same a11y + dark-mode baseline. Shared UI primitives (`Icon`, `LayoutStateService`, brand tokens) graduate to `libs/shared/*` as both apps need them — see [ADR-0020](docs/decisions/0020-portal-admin-app.md).
 - **Local quality gates:** Husky + lint-staged + commitlint with Conventional Commits — see [ADR-0007](docs/decisions/0007-pre-commit-hooks-and-conventional-commits.md).
 - **Runtime:** Node.js latest LTS major.