Backend: Authentik directory client + endpoint (Slice 2)

AuthentikClient (clients/authentik.py) wraps Authentik's directory API:
- Bearer-token requests.Session, base_url normalization (rstrip / and
  trailing /api/v3), get() helper mirroring JellyseerrClient.
- users(search, page, page_size) calls GET /api/v3/core/users/ and
  normalizes Authentik's {pagination, results} shape into
  {items, total, page, page_size} for frontend consumption.

Directory endpoint (routers/authentik_users.py):
- GET /api/services/authentik/{service_id}/users resolves the service
  record, builds the client from decrypted api_token, returns the
  normalized user list.
- Graceful error handling matching monitoring.py: not-configured and
  unreachable return {items:[], total:0, error} with 200 (no 500s).
- _resolve_service_record copied in (self-contained; shared-utility
  extraction is a follow-up).

Router registered in main.py.

Tests: 12 new (8 client unit + 4 endpoint integration covering success,
not-configured, unreachable, URL/params). 268 backend tests pass; ruff
clean.

Refs openspec/changes/services-as-hub-ia/ (spec R6.2/R7.2, tasks slice 2).

backend/src/media_library_viewer_api/main.py

@@ -21,6 +21,9 @@ from media_library_viewer_api.observability import (
     record_request,
     set_current_request_id,
 )
+from media_library_viewer_api.routers import (
+    authentik_users as authentik_users_router,
+)
 from media_library_viewer_api.routers import backups as backups_router
 from media_library_viewer_api.routers import dashboard, files, jobs, media, monitoring, tasks, users
 from media_library_viewer_api.routers import services as services_router
@@ -142,6 +145,7 @@ app.include_router(settings_router)
 app.include_router(backups_router.router)
 app.include_router(widgets_router.router)
 app.include_router(services_router.router)
+app.include_router(authentik_users_router.router)
 
 
 @app.get("/api/health")