Fix async context manager usage in BackgroundLoaderService

- Changed 'async for' to 'async with' for get_db_session()
- get_db_session() is @asynccontextmanager, requires async with not async for
- Created 5 comprehensive unit tests verifying the fix
- All tests pass, background loading now works correctly

src/server/services/background_loader_service.py

@@ -279,7 +279,7 @@ class BackgroundLoaderService:
             from src.server.database.connection import get_db_session
             from src.server.database.service import AnimeSeriesService
             
-            async for db in get_db_session():
+            async with get_db_session() as db:
                 try:
                     # Check what data is missing
                     missing = await self.check_missing_data(
@@ -337,8 +337,6 @@ class BackgroundLoaderService:
                     # Broadcast error
                     await self._broadcast_status(task)
                 
-                break  # Exit async for loop after first iteration
-                
         finally:
             # Remove from active tasks
             self.active_tasks.pop(task.key, None)