fix: use worker_tasks list instead of non-existent worker_task attribute

src/server/api/anime.py

@@ -834,9 +834,9 @@ async def add_series(
         # 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()
+            loader_running = bool(
+                background_loader.worker_tasks
+                and any(not t.done() for t in background_loader.worker_tasks)
             )
             if (
                 not loader_running