Fix: Use yt_dlp.utils.DownloadCancelled for proper download cancellation

- Import and use DownloadCancelled exception which YT-DLP properly handles
- Add InterruptedError handling throughout the call chain
- Fire 'cancelled' status event when download is cancelled
- Handle InterruptedError in DownloadService to set CANCELLED status

src/server/services/anime_service.py

@@ -846,6 +846,7 @@ class AnimeService:
             
         Raises:
             AnimeServiceError: If download fails
+            InterruptedError: If download was cancelled
             
         Note:
             The 'key' parameter is the primary identifier used for all
@@ -864,6 +865,10 @@ class AnimeService:
                 key=key,
                 item_id=item_id,
             )
+        except InterruptedError:
+            # Download was cancelled - re-raise for proper handling
+            logger.info("Download cancelled, propagating cancellation")
+            raise
         except Exception as exc:
             logger.exception("download failed")
             raise AnimeServiceError("Download failed") from exc