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/tests/test_services.py

@@ -101,7 +101,12 @@ def test_authentik_service_definition():
 def test_definitions_declare_widget_kinds():
     assert {wk.kind for wk in get_service_definition("prometheus").widget_kinds} == {"metric", "chart", "gauge", "mean"}
     assert {wk.kind for wk in get_service_definition("alertmanager").widget_kinds} == {"active_alerts"}
-    assert {wk.kind for wk in get_service_definition("jellyfin").widget_kinds} == {"activity", "now_playing"}
+    assert {wk.kind for wk in get_service_definition("jellyfin").widget_kinds} == {
+        "activity",
+        "now_playing",
+        "stat",
+        "stats_overview",
+    }
     assert get_service_definition("nextcloud").widget_kinds == []
     assert get_service_definition("authentik").widget_kinds == []
     assert {wk.kind for wk in get_service_definition("backups").widget_kinds} == {"summary"}