feat: add 'missing_from_fs' property to various schemas and update related logic for better tracking of missing files

backend/handler/database/saves_handler.py

@@ -90,7 +90,7 @@ class DBSavesHandler(DBBaseHandler):
                     Save.file_name.not_in(saves_to_keep),
                 )
             )
-            .values(**{"missing": True})
+            .values(**{"missing_from_fs": True})
             .execution_options(synchronize_session="evaluate")
         )