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>

tests/unit/test_serie_scanner.py

@@ -75,12 +75,12 @@ class TestSerieScannerInitialization:
 class TestSerieScannerScan:
     """Test file-based scan operations."""
 
-    def test_file_based_scan_works(
+    def test_scan_persists_to_db(
         self, temp_directory, mock_loader, sample_serie
     ):
-        """Test file-based scan works properly."""
+        """Test scan persists series to database."""
         scanner = SerieScanner(temp_directory, mock_loader)
-        
+
         with patch.object(scanner, 'get_total_to_scan', return_value=1):
             with patch.object(
                 scanner,
@@ -100,12 +100,15 @@ class TestSerieScannerScan:
                         return_value=({1: [2, 3]}, "aniworld.to")
                     ):
                         with patch.object(
-                            sample_serie, 'save_to_file'
-                        ) as mock_save:
+                            scanner, '_persist_serie_to_db'
+                        ) as mock_persist:
                             scanner.scan()
-                            
-                            # Verify file was saved
-                            mock_save.assert_called_once()
+
+                            # Verify DB persistence was called
+                            mock_persist.assert_called_once()
+                            # Check the serie passed matches
+                            call_args = mock_persist.call_args
+                            assert call_args[0][0].key == "attack-on-titan"
 
     def test_keydict_populated_after_scan(
         self, temp_directory, mock_loader, sample_serie