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/CHANGELOG.md

@@ -139,6 +139,10 @@ This changelog follows [Keep a Changelog](https://keepachangelog.com/) principle
 - Modified `src/server/api/anime.py` to save scanned episodes to database
 - Episodes table properly tracks missing episodes with automatic cleanup
 
+### Deprecated
+
+- **Legacy Series Files (key/data)**: File-based series storage is deprecated. `key` and `data` files in anime folders will be removed in v3.0.0. Database storage is now the primary method. See [docs/MIGRATION_GUIDE.md](docs/MIGRATION_GUIDE.md) for details.
+
 ---
 
 ## Sections for Each Release