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/widgets.py

@@ -33,6 +33,7 @@ from media_library_viewer_api.widgets.sources import (
     build_service_record,
     get_builtin_adapter,
     get_service_adapter,
+    get_stats_adapter,
 )
 
 
@@ -199,7 +200,11 @@ async def fetch_data(
                 error="Service is disabled",
                 fetched_at=int(time.time()),
             ).model_dump()
-        adapter = get_service_adapter(service_row["service_type"])
+        adapter = (
+            get_stats_adapter()
+            if widget_kind in ("stat", "stats_overview")
+            else get_service_adapter(service_row["service_type"])
+        )
         if adapter is None:
             return WidgetDataResponse(
                 widget_id=widget_id,