feat: add legacy key/data file migration to database

- Add migration_legacy_files_completed flag to SystemSettings model
- Create legacy_file_migration service to migrate series from key/data files
- Integrate legacy migration into initialization_service startup flow
- Add integration tests for legacy file migration
- Update DATABASE.md documentation with migration details
- Fix various test and service issues (nfo_repair, tmdb_client, download_service)
- Add test_database_schema unit tests

src/server/services/download_service.py

@@ -1122,6 +1122,7 @@ class DownloadService:
                 item.status = DownloadStatus.PENDING
                 item.error = None
                 item.progress = None
+                item.retry_count += 1
                 self._add_to_pending_queue(item)
                 retried_ids.append(item.id)