Fix unused imports (ruff lint)

Remove unused mimetypes imports from objectref.py and storage.py,
remove unused Path import and generate_token from staged_insert.py,
and fix f-string without placeholders in objectref.py.

Author: claude

src/datajoint/objectref.py

@@ -7,11 +7,10 @@
 """
 
 import json
-import mimetypes
 from dataclasses import dataclass
 from datetime import datetime
 from pathlib import Path
-from typing import IO, Any, Iterator
+from typing import IO, Iterator
 
 import fsspec
 
@@ -345,7 +344,7 @@ def _verify_folder(self) -> bool:
                     errors.append(f"Size mismatch for {file_info['path']}: expected {expected_size}, got {actual_size}")
 
         if errors:
-            raise IntegrityError(f"Folder verification failed:\n" + "\n".join(errors))
+            raise IntegrityError("Folder verification failed:\n" + "\n".join(errors))
 
         return True
 

src/datajoint/staged_insert.py

@@ -9,14 +9,13 @@
 import mimetypes
 from contextlib import contextmanager
 from datetime import datetime, timezone
-from pathlib import Path
 from typing import IO, Any
 
 import fsspec
 
 from .errors import DataJointError
 from .settings import config
-from .storage import StorageBackend, build_object_path, generate_token
+from .storage import StorageBackend, build_object_path
 
 
 class StagedInsert:

src/datajoint/storage.py

@@ -7,7 +7,6 @@
 
 import json
 import logging
-import mimetypes
 import secrets
 import urllib.parse
 from datetime import datetime, timezone