Fix: Add graceful download cancellation on Ctrl+C

- Add cancellation flag to AniworldLoader with request_cancel/reset_cancel/is_cancelled methods
- Update base_provider.Loader interface with cancellation abstract methods
- Integrate cancellation check in YT-DLP progress hooks
- Add request_download_cancel method to SeriesApp and AnimeService
- Update DownloadService.stop() to request cancellation before shutdown
- Clean up temp files on cancellation

src/server/services/anime_service.py

@@ -72,6 +72,21 @@ class AnimeService:
             logger.exception("Failed to subscribe to SeriesApp events")
             raise AnimeServiceError("Initialization failed") from e
 
+    def request_download_cancel(self) -> None:
+        """Request cancellation of any ongoing download.
+        
+        This method signals the underlying download provider to stop
+        any active downloads. The cancellation happens asynchronously
+        via progress hooks in the downloader.
+        
+        Should be called during shutdown to stop in-progress downloads.
+        """
+        logger.info("Requesting download cancellation via AnimeService")
+        try:
+            self._app.request_download_cancel()
+        except Exception as e:
+            logger.warning("Failed to request download cancellation: %s", e)
+
     def _on_download_status(self, args) -> None:
         """Handle download status events from SeriesApp.