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/utils/dependencies.py

@@ -27,6 +27,7 @@ logger = logging.getLogger(__name__)
 
 if TYPE_CHECKING:
     from src.server.services.anime_service import AnimeService
+    from src.server.services.background_loader_service import BackgroundLoaderService
     from src.server.services.download_service import DownloadService
 
 # Security scheme for JWT authentication
@@ -40,6 +41,7 @@ _series_app: Optional[SeriesApp] = None
 # Global service instances
 _anime_service: Optional["AnimeService"] = None
 _download_service: Optional["DownloadService"] = None
+_background_loader_service: Optional["BackgroundLoaderService"] = None
 
 
 @dataclass
@@ -452,3 +454,51 @@ def reset_download_service() -> None:
     """Reset global DownloadService instance (for testing/config changes)."""
     global _download_service
     _download_service = None
+
+
+def get_background_loader_service() -> "BackgroundLoaderService":
+    """
+    Dependency to get BackgroundLoaderService instance.
+    
+    Returns:
+        BackgroundLoaderService: The background loader service for async data loading
+        
+    Raises:
+        HTTPException: If BackgroundLoaderService initialization fails
+    """
+    global _background_loader_service
+    
+    if _background_loader_service is None:
+        try:
+            from src.server.services.background_loader_service import (
+                BackgroundLoaderService,
+            )
+            from src.server.services.websocket_service import get_websocket_service
+
+            anime_service = get_anime_service()
+            series_app = get_series_app()
+            websocket_service = get_websocket_service()
+            
+            _background_loader_service = BackgroundLoaderService(
+                websocket_service=websocket_service,
+                anime_service=anime_service,
+                series_app=series_app
+            )
+        except HTTPException:
+            raise
+        except Exception as e:
+            raise HTTPException(
+                status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+                detail=(
+                    "Failed to initialize BackgroundLoaderService: "
+                    f"{str(e)}"
+                ),
+            ) from e
+    
+    return _background_loader_service
+
+
+def reset_background_loader_service() -> None:
+    """Reset global BackgroundLoaderService instance (for testing/config changes)."""
+    global _background_loader_service
+    _background_loader_service = None