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/jellyseerr.py

@@ -0,0 +1,52 @@
+"""Jellyseerr request stats — powers the Requests tab on the Jellyfin page.
+
+Jellyseerr is an optional companion of the Jellyfin service. This router
+resolves the Jellyfin service instance (by ``jellyfin_service_id`` or the first
+enabled one) and delegates to the registered Jellyseerr stats provider, which
+shares its short-TTL cache with the ``stat`` / ``stats_overview`` widgets so
+the tab and the widgets don't each hit Jellyseerr.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from fastapi import APIRouter, Depends, HTTPException
+
+from media_library_viewer_api.dependencies import get_settings_store
+from media_library_viewer_api.services.service_resolution import resolve_service_record
+from media_library_viewer_api.services.settings_store import SettingsStore
+from media_library_viewer_api.widgets import jellyseerr_stats  # noqa: F401 — ensure provider registration
+from media_library_viewer_api.widgets.stats_provider import get_stats_provider
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/jellyseerr", tags=["jellyseerr"])
+
+
+def _serialize(result) -> dict:
+    return {
+        "stats": [{"key": s.key, "label": s.label, "value": s.value} for s in result.stats],
+        "recent": result.recent,
+        "detail": result.detail,
+    }
+
+
+@router.get("/stats")
+def get_jellyseerr_stats(
+    jellyfin_service_id: str | None = None,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict:
+    """Return Jellyseerr request counts + a recent-requests list."""
+    service = resolve_service_record(store, "jellyfin", jellyfin_service_id)
+    if service is None:
+        raise HTTPException(status_code=503, detail="No Jellyfin service is configured.")
+    provider = get_stats_provider("jellyfin")
+    if provider is None:  # pragma: no cover - registered at import
+        raise HTTPException(status_code=503, detail="Jellyseerr stats provider is not available.")
+    try:
+        result = provider.fetch_stats(service)
+    except Exception as exc:  # pragma: no cover - provider guards internally
+        logger.exception("Jellyseerr stats endpoint failed")
+        raise HTTPException(status_code=502, detail=f"Jellyseerr fetch failed: {exc}") from exc
+    return _serialize(result)