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/DATABASE.md

@@ -588,7 +588,52 @@ EpisodeService.create(is_downloaded=False)
 
 ---
 
-## 13. Database Location
+## 13. Series Persistence
+
+### Schema
+
+**AnimeSeries Table**: Stores series metadata (key, name, site, folder, year)
+
+| Column    | Type         | Constraints               | Description           |
+|-----------|--------------|---------------------------|----------------------|
+| `id`      | INTEGER      | PRIMARY KEY               | Auto-increment       |
+| `key`     | VARCHAR(255) | UNIQUE, NOT NULL          | Series provider key   |
+| `name`    | VARCHAR(500) | NOT NULL                 | Display name         |
+| `site`    | VARCHAR(500) |                           | Provider site URL    |
+| `folder`  | VARCHAR(1000)|                           | Filesystem folder    |
+
+**Episode Table**: Stores per-episode metadata (season, episode_number, is_downloaded)
+
+| Column          | Type         | Constraints               | Description           |
+|-----------------|--------------|---------------------------|----------------------|
+| `id`            | INTEGER      | PRIMARY KEY               | Auto-increment       |
+| `series_id`     | INTEGER      | FOREIGN KEY → anime_series| Parent series        |
+| `season`        | INTEGER      | NOT NULL                  | Season number        |
+| `episode_number`| INTEGER      | NOT NULL                  | Episode number       |
+| `is_downloaded` | BOOLEAN      | DEFAULT FALSE            | Download status      |
+
+### Relationships
+
+- `AnimeSeries.episodes` → List of Episode objects (one-to-many)
+- `Episode.series` → Parent AnimeSeries (many-to-one)
+- Cascade delete: Deleting a series removes all its episodes
+
+### Queries
+
+```python
+# Get all series with episodes
+AnimeSeriesService.get_all(db, with_episodes=True)
+
+# Get by provider key
+AnimeSeriesService.get_by_key(db, key)
+
+# Get by folder path
+AnimeSeriesService.get_by_folder(db, folder)
+```
+
+---
+
+## 14. Database Location
 
 | Environment | Default Location                                  |
 | ----------- | ------------------------------------------------- |