Remove legacy key file support after DB migration

- SerieScanner: Remove key file fallback, keep data file fallback
- SystemSettings: Add legacy_key_cleanup_completed flag
- initialization_service: Add cleanup task to remove key files from folders with DB entries
- Tests updated to reflect key file removal from legacy path

Key files caused duplicate key errors on folder rename. DB is now sole source of truth.

tests/unit/test_system_settings_service.py

@@ -23,6 +23,8 @@ async def test_system_settings_integration():
         assert settings.initial_scan_completed is False
         assert settings.initial_nfo_scan_completed is False
         assert settings.initial_media_scan_completed is False
+        assert settings.migration_legacy_files_completed is False
+        assert settings.legacy_key_cleanup_completed is False
         
     # Test checking individual flags
     async with get_db_session() as db:
@@ -34,6 +36,12 @@ async def test_system_settings_integration():
         
         is_media_done = await SystemSettingsService.is_initial_media_scan_completed(db)
         assert is_media_done is False
+        
+        is_migration_done = await SystemSettingsService.is_migration_legacy_files_completed(db)
+        assert is_migration_done is False
+        
+        is_key_cleanup_done = await SystemSettingsService.is_legacy_key_cleanup_completed(db)
+        assert is_key_cleanup_done is False
     
     # Test marking scans as completed
     async with get_db_session() as db:
@@ -56,6 +64,8 @@ async def test_system_settings_integration():
         assert settings.initial_scan_completed is False
         assert settings.initial_nfo_scan_completed is False
         assert settings.initial_media_scan_completed is False
+        assert settings.migration_legacy_files_completed is False
+        assert settings.legacy_key_cleanup_completed is False
 
 
 if __name__ == "__main__":