Fix Download to Library crashing with a NameError

run_download_task sets e621_match_status from MediaItem, but the module
only imported DownloadTask — every download that carried e621 metadata
failed right after indexing, leaving the file in the library unlinked.
Verified the runner end-to-end with a stubbed fetch.

backend/apps/library/downloads.py

@@ -12,7 +12,7 @@ from django.db import connection
 from django.utils import timezone
 
 from . import services
-from .models import DownloadTask
+from .models import DownloadTask, MediaItem
 
 logger = logging.getLogger(__name__)