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

docs/DEVELOPMENT.md

@@ -398,3 +398,29 @@ forward this to `notification_service.notify_download_failed()` so users see
 a HIGH-priority alert. The loader keeps the failure detail in
 `logs/download_errors.log` for post-mortem.
 
+## Series Storage
+
+### Overview
+
+Series metadata now stored in the database (SQLAlchemy ORM).
+Legacy files (`key` and `data` per folder) are deprecated but preserved
+for backward compatibility.
+
+### Architecture
+
+- **Database**: Single source of truth for all series metadata
+- **In-Memory Cache**: SeriesApp maintains a cache for performance
+- **Filesystem**: Only used for episode files themselves, not metadata
+
+### Migration
+
+First startup after upgrade automatically imports any legacy
+series files into the database.
+
+### Legacy Files
+
+- `key` file: Contains series provider key (deprecated)
+- `data` file: Contains Serie JSON object (deprecated)
+
+Both are safe to delete after migration; not needed for normal operation.
+