Implement async series data loading with background processing

- Add loading status fields to AnimeSeries model
- Create BackgroundLoaderService for async task processing
- Update POST /api/anime/add to return 202 Accepted immediately
- Add GET /api/anime/{key}/loading-status endpoint
- Integrate background loader with startup/shutdown lifecycle
- Create database migration script for loading status fields
- Add unit tests for BackgroundLoaderService (10 tests, all passing)
- Update AnimeSeriesService.create() to accept loading status fields

Architecture follows clean separation with no code duplication:
- BackgroundLoader orchestrates, doesn't reimplement
- Reuses existing AnimeService, NFOService, WebSocket patterns
- Database-backed status survives restarts

src/server/database/service.py

@@ -65,6 +65,11 @@ class AnimeSeriesService:
         site: str,
         folder: str,
         year: int | None = None,
+        loading_status: str = "completed",
+        episodes_loaded: bool = True,
+        logo_loaded: bool = False,
+        images_loaded: bool = False,
+        loading_started_at: datetime | None = None,
     ) -> AnimeSeries:
         """Create a new anime series.
         
@@ -75,6 +80,11 @@ class AnimeSeriesService:
             site: Provider site URL
             folder: Local filesystem path
             year: Release year (optional)
+            loading_status: Initial loading status (default: "completed")
+            episodes_loaded: Whether episodes are loaded (default: True for backward compat)
+            logo_loaded: Whether logo is loaded (default: False)
+            images_loaded: Whether images are loaded (default: False)
+            loading_started_at: When loading started (optional)
         
         Returns:
             Created AnimeSeries instance
@@ -88,6 +98,11 @@ class AnimeSeriesService:
             site=site,
             folder=folder,
             year=year,
+            loading_status=loading_status,
+            episodes_loaded=episodes_loaded,
+            logo_loaded=logo_loaded,
+            images_loaded=images_loaded,
+            loading_started_at=loading_started_at,
         )
         db.add(series)
         await db.flush()