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/routers/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src/media_library_viewer_api/routers
 
 ## role
-FastAPI router package that defines all HTTP API endpoints for the media library viewer backend, organizing routes by domain (auth, backups, dashboards, files, jobs, media, monitoring, services, settings, tasks, widgets).
+FastAPI router package that organizes and exposes all REST API endpoints for the media library viewer backend, covering authentication, dashboards, media management, backups, monitoring, services, files, and task execution.
 ## parent
 index: backend/src/media_library_viewer_api/.pi-map.index.md
 map: backend/src/media_library_viewer_api/.pi-map.md
@@ -15,6 +15,7 @@ map: backend/src/media_library_viewer_api/.pi-map.md
 - dashboard.py
 - dashboards.py
 - files.py
+- jellyseerr.py
 - jobs.py
 - media.py
 - monitoring.py