Fix stale data file updates on download completion

When episodes are downloaded successfully, the in-memory Serie.episodeDict
is updated, but the deprecated data file was not being synced. This caused
UI to show episodes as missing when already downloaded.

Changes:
- Update data file in _remove_episode_from_memory when download completes
- DB is authoritative; data file is optional backup (deprecated)
- Gracefully skip update if data file doesn't exist

New integration tests for episode download sync:
- Verify episode removed from missing list after download
- Verify in-memory cache updated after download
- Verify data file updated after download (when it exists)
- Verify downloads work without data file

src/server/services/download_service.py

@@ -466,6 +466,27 @@ class DownloadService:
                     "missing episodes remaining",
                     len(app.series_list),
                 )
+                
+                # Update deprecated data file if it exists
+                # DB is authoritative; data file is optional backup
+                serie_folder = serie.folder
+                data_path = Path(self._directory) / serie_folder / "data"
+                if data_path.exists():
+                    try:
+                        import warnings
+                        with warnings.catch_warnings():
+                            warnings.simplefilter("ignore", DeprecationWarning)
+                            serie.save_to_file(str(data_path))
+                        logger.debug(
+                            "Updated data file after download: %s",
+                            data_path,
+                        )
+                    except Exception as e:
+                        logger.warning(
+                            "Failed to update data file %s: %s",
+                            data_path,
+                            e,
+                        )
             else:
                 logger.debug(
                     "Episode %d not in season %d for %s, "