feat(SerieScanner): add warning event for duplicate series keys

- Add on_warning event system with subscribe/unsubscribe methods
- Change duplicate key handling from error to warning
- Fire on_warning event when duplicate series detected
- Include metadata: key, duplicate_folder, existing_folder

src/server/services/folder_rename_service.py

@@ -115,6 +115,29 @@ def _is_series_being_downloaded(series_folder: str) -> bool:
         return True
 
 
+def _cleanup_stale_files_after_rename(new_path: Path, new_name: str) -> None:
+    """Remove legacy 'key' file after successful folder rename.
+
+    Also checks for orphaned folders with the same key that may have been
+    left behind from previous rename operations.
+
+    Args:
+        new_path: The new folder path after rename.
+        new_name: The new folder name.
+    """
+    key_file = new_path / "key"
+    if key_file.exists():
+        try:
+            key_file.unlink()
+            logger.info(
+                "Removed legacy 'key' file after rename: %s", key_file
+            )
+        except OSError as exc:
+            logger.warning(
+                "Could not remove legacy 'key' file %s: %s", key_file, exc
+            )
+
+
 async def _update_database_paths(
     old_folder: str,
     new_folder: str,
@@ -315,6 +338,9 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
             # Update database records
             await _update_database_paths(current_name, expected_name, anime_dir)
 
+            # Clean up stale/legacy files after successful rename
+            _cleanup_stale_files_after_rename(expected_path, expected_name)
+
         except PermissionError as exc:
             logger.error(
                 "Permission denied renaming '%s' → '%s': %s",