feat(scanner): replace file writes with DB persistence for series

- SerieScanner.scan() now calls _persist_serie_to_db() instead of serie.save_to_file()
- Added _sync_episodes_to_db() helper to handle episode CRUD during sync
- EpisodeService gains delete_by_series() for targeted episode deletion
- SerieList gains add_to_db() async method for DB-based series addition
- test_serie_scanner_db_writes.py covers create/update/preserve/sync scenarios
- DATABASE.md updated with Series Persistence Flow section

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/DATABASE.md

@@ -509,6 +509,85 @@ After verifying database schema supports all fields, file-based storage can be r
 
 ---
 
+## 12. Series Persistence Flow
+
+When a directory scan discovers or updates series, the scanner persists data to the database instead of writing to disk files.
+
+### Scan Flow
+
+```
+Scan Directory
+    │
+    ▼
+Find MP4 Files → Extract Serie Key
+    │
+    ▼
+Check DB for Existing Series (by key)
+    │
+    ├─── EXISTS ──────────────────────► Update Series Metadata
+    │                                        │
+    │                                        ▼
+    │                                 Sync Episodes to DB
+    │                                      │
+    │◄──────────────────────────────────────┘
+    │
+    └─── NEW ───────────────────────────► Create New Series Record
+                                             │
+                                             ▼
+                                      Create Episode Records
+                                             │
+                                             ▼
+                                      Return to Scan Loop
+```
+
+### Key Methods
+
+**SerieScanner._persist_serie_to_db()**
+- Called after `get_missing_episodes_and_season()` computes episodeDict
+- Uses `AnimeSeriesService.get_by_key()` to check if series exists
+- If exists: calls `AnimeSeriesService.update()` + `_sync_episodes_to_db()`
+- If new: calls `AnimeSeriesService.create()` + creates episodes
+
+**SerieScanner._sync_episodes_to_db()**
+- Gets existing episodes from DB via `EpisodeService.get_by_series()`
+- Compares with new episodeDict
+- Removes episodes no longer missing (unless `is_downloaded=True`)
+- Adds new missing episodes
+- Preserves `is_downloaded=True` episodes when removing missing ones
+
+**SerieList.add_to_db()**
+- Used when adding a new discovered series via API
+- Creates filesystem folder + database record + episode records
+
+### Episode Sync Logic
+
+```python
+# For each episode in DB but not in new episodeDict:
+if episode.is_downloaded:
+    # Keep - file exists, don't remove
+    pass
+else:
+    # Remove - no longer missing
+    EpisodeService.delete()
+
+# For each episode in new episodeDict but not in DB:
+# Add as new missing episode
+EpisodeService.create(is_downloaded=False)
+```
+
+### Transaction Handling
+
+- DB operations use their own session with commit/rollback
+- If DB write fails, error is logged and scan continues
+- File-based `save_to_file()` no longer called during scan
+
+### Migration Path
+
+1. v2.x: Scanner writes to both DB (primary) and files (fallback)
+2. v3.0: Scanner writes only to DB, file methods removed
+
+---
+
 ## 13. Database Location
 
 | Environment | Default Location                                  |