✨(backend) add command to clean pending and deleted files

Since we have added custom backgrounds, they could be
soft deleted but were never really deleted.
This commits introduces a command to actually delete
this files.
In introduces also a command to purge pending upload
.

src/backend/core/api/viewsets.py

@@ -1187,7 +1187,10 @@ class FileViewSet(
         serializer.save(creator=self.request.user)
 
     def perform_destroy(self, instance):
-        """Override to implement a soft delete instead of dumping the record in database."""
+        """Override to implement a soft delete instead of dumping the record in database.
+
+        Files are actually purged by commands that should run periodically.
+        """
         instance.soft_delete()
 
     @decorators.action(detail=True, methods=["post"], url_path="upload-ended")