Add sync_single_series_after_scan with NFO metadata and WebSocket updates

- Implement sync_single_series_after_scan to persist scanned series to database
- Enhanced _broadcast_series_updated to include full NFO metadata (nfo_created_at, nfo_updated_at, tmdb_id, tvdb_id)
- Add immediate episode scanning in add_series endpoint when background loader isn't running
- Implement updateSingleSeries in frontend to handle series_updated WebSocket events
- Add SERIES_UPDATED event constant to WebSocket event definitions
- Update background loader to use sync_single_series_after_scan method
- Simplified background loader initialization in FastAPI app
- Add comprehensive tests for series update WebSocket payload and episode counting logic
- Import reorganization: move get_background_loader_service to dependencies module

src/server/api/anime.py

@@ -15,12 +15,10 @@ from src.server.exceptions import (
     ValidationError,
 )
 from src.server.services.anime_service import AnimeService, AnimeServiceError
-from src.server.services.background_loader_service import (
-    BackgroundLoaderService,
-    get_background_loader_service,
-)
+from src.server.services.background_loader_service import BackgroundLoaderService
 from src.server.utils.dependencies import (
     get_anime_service,
+    get_background_loader_service,
     get_optional_database_session,
     get_series_app,
     require_auth,
@@ -641,6 +639,7 @@ async def add_series(
     request: AddSeriesRequest,
     _auth: dict = Depends(require_auth),
     series_app: Any = Depends(get_series_app),
+    anime_service: AnimeService = Depends(get_anime_service),
     db: Optional[AsyncSession] = Depends(get_optional_database_session),
     background_loader: BackgroundLoaderService = Depends(get_background_loader_service),
 ) -> dict:
@@ -831,8 +830,44 @@ async def add_series(
                 key,
                 e
             )
+
+        # Step F: Scan missing episodes immediately if background loader is not running
+        # Uses existing SerieScanner and AnimeService sync to avoid duplicates
+        try:
+            loader_running = (
+                background_loader.worker_task is not None
+                and not background_loader.worker_task.done()
+            )
+            if (
+                not loader_running
+                and series_app
+                and hasattr(series_app, "serie_scanner")
+            ):
+                missing_episodes = series_app.serie_scanner.scan_single_series(
+                    key=key,
+                    folder=folder
+                )
+                total_missing = sum(
+                    len(eps) for eps in missing_episodes.values()
+                )
+                logger.info(
+                    "Scanned %d missing episodes for %s",
+                    total_missing,
+                    key
+                )
+                
+                # Persist scan results to database (includes episodes)
+                # scan_single_series updates serie_scanner.keyDict with episodeDict
+                # sync_single_series_after_scan retrieves from there and saves to DB
+                await anime_service.sync_single_series_after_scan(key)
+        except Exception as e:
+            logger.warning(
+                "Failed to scan missing episodes for %s: %s",
+                key,
+                e
+            )
         
-        # Step F: Return immediate response (202 Accepted)
+        # Step G: Return immediate response (202 Accepted)
         response = {
             "status": "success",
             "message": f"Series added successfully: {name}. Data will be loaded in background.",