feat(jellyseer): stats backend — provider, stat widgets, router (slice 1/3)

Groundwork for Jellyseerr request stats in the Jellyfin service, behind a
small reusable abstraction so future stats services (Sonarr/Radarr) reuse it.

Backend:
- JellyseerrClient.request_count() -> /api/v1/request/count (normalized
  total/pending/approved/declined/processing/available) and recent_requests()
  -> /api/v1/request mapped to {name,type,status,media_status,created_at}
  with numeric status enums labelled.
- widgets/stats_provider.py: StatsProvider protocol + registry keyed by
  service_type (StatValue/StatsResult). A thin generic interface.
- widgets/jellyseerr_stats.py: JellyseerrStatsProvider registered for the
  Jellyfin service; reuses one authenticated client per service (lru_cache) and
  caches the StatsResult for ~10s under a lock, so multiple widgets + the tab
  collapse onto one Jellyseerr fetch (same lesson as the qBittorrent client).
  Accepts jellyseerr_api_key from secrets OR config during the upcoming
  config->secret migration.
- Jellyfin service gains two widget kinds: `stat` (a Literal selector over the
  six stats — the "extract one value into a widget" affordance) and
  `stats_overview` (all stats + recent list).
- widgets router routes widget_kind in {stat, stats_overview} to a generic
  StatsWidgetSource (dispatches to the service type's provider), independent of
  service type.
- new /api/jellyseerr/stats router endpoint for the Requests tab (resolves the
  Jellyfin service by id or first-enabled; shares the provider cache).

Tests: provider normalization, not-configured, TTL caching; stat selector +
overview + unknown-stat widget dispatch; 7 new tests. 400/400 backend pass;
ruff clean.

backend/src/media_library_viewer_api/main.py

@@ -27,6 +27,7 @@ from media_library_viewer_api.routers import (
 from media_library_viewer_api.routers import backups as backups_router
 from media_library_viewer_api.routers import dashboard, files, jobs, media, monitoring, tasks
 from media_library_viewer_api.routers import dashboards as dashboards_router
+from media_library_viewer_api.routers import jellyseerr as jellyseerr_router
 from media_library_viewer_api.routers import services as services_router
 from media_library_viewer_api.routers import widgets as widgets_router
 from media_library_viewer_api.routers.settings import router as settings_router
@@ -169,6 +170,7 @@ app.include_router(settings_router)
 app.include_router(backups_router.router)
 app.include_router(widgets_router.router)
 app.include_router(dashboards_router.router)
+app.include_router(jellyseerr_router.router)
 app.include_router(services_router.router)
 app.include_router(authentik_users_router.router)