fix: remove invalid updateMany option in moveMultipleFiles

{ new: true } only applies to findOneAndUpdate, not updateMany - it was
always a silent no-op here. Harmless until a stricter mongoose/mongodb
type resolution (no lockfile is committed, so installs float) rejected
it outright, breaking `npm run build` inside the Docker image.

backend/db/mongoDB/fileDB.ts

@@ -357,8 +357,7 @@ class DbUtil implements IFileDB {
           "metadata.parent": newParent,
           "metadata.parentList": newParentList,
         },
-      },
-      { new: true }
+      }
     );
   };