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

docs/architecture.md

@@ -84,7 +84,7 @@ flowchart TB
 
 Notes embedded in the diagram:
 
-- **Two SPAs, one BFF.** `portal-shell` is the end-user surface; `portal-admin` is a separate Angular app on a distinct origin with its own bundle, sign-in flow, and session cookie (per ADR-0020). They share libs but never a runtime — an admin session is not silently authenticated to `portal-shell` and vice versa. The admin entry route is gated by an Entra `admin` app-role claim + `@RequireMfa({ freshness: 600 })` per ADR-0011.
+- **Two SPAs, one BFF.** `portal-shell` is the end-user surface; `portal-admin` is a separate Angular app on a distinct origin with its own bundle, sign-in flow, and session cookie (per ADR-0020). They share libs but never a runtime — an admin session is not silently authenticated to `portal-shell` and vice versa. The admin entry route is gated by an Entra `Portal.Admin` app-role claim + `@RequireMfa({ freshness: 600 })` per ADR-0011.
 - The SPAs carry no token. Only the opaque session id cookie (`__Host-portal_session` / `__Host-portal_admin_session`) plus the CSRF cookie (`__Host-portal_csrf`, read by JS for double-submit).
 - `traceparent` (W3C) propagates from the browser through the BFF to the downstream APIs — same `trace_id` end-to-end.
 - The OTel Collector is the only piece coupled to the eventual on-prem observability backend. Choice of backend (Grafana Loki + Tempo, ELK, …) is deferred to the on-prem infrastructure ADR.