fix: prevent duplicate series when same anime key exists in different folder

- Add check for existing series by key in SetupService.run to skip duplicates
- Fix Path construction in initialization_service.py cleanup function
- Update unit tests to mock get_by_key and get_series_app

src/server/services/setup_service.py

@@ -378,6 +378,18 @@ class SetupService:
                         )
                         continue
                     
+                    # Also check if a series with this key already exists (different folder, same anime)
+                    existing_by_key = await AnimeSeriesService.get_by_key(db, resolved_key)
+                    if existing_by_key:
+                        logger.debug(
+                            "Series with key already exists, skipping",
+                            folder=folder_name,
+                            key=resolved_key,
+                            existing_folder=existing_by_key.folder
+                        )
+                        skipped_existing += 1
+                        continue
+                    
                     # Check filesystem properties
                     props = cls._get_series_properties(folder)