fix: validate generated keys before creating Serie objects

- Add is_valid_key check in SerieScanner._read_data_from_file() to prevent
  passing invalid keys to Serie constructor (caused ValueError)
- Improve error message for key generation failures
- Add warning log before removing duplicate source folders in rename service

src/server/services/folder_rename_service.py

@@ -602,6 +602,11 @@ async def validate_and_rename_series_folders(dry_run: bool = False) -> Dict[str,
             try:
                 import shutil
 
+                logger.warning(
+                    "Removing source duplicate folder '%s' — target '%s' already exists",
+                    current_name,
+                    expected_name,
+                )
                 shutil.rmtree(series_dir)
                 logger.info(
                     "Removed source folder '%s' — series already exists at target",