feat: rescan now saves to database instead of data files

- Update SeriesApp.rescan() to use database storage by default (use_database=True)
- Use SerieScanner.scan_async() for database mode, which saves directly to DB
- Fall back to legacy file-based scan() when use_database=False (for CLI compatibility)
- Reinitialize SerieList from database after scan when in database mode
- Update unit tests to use use_database=False for mocked tests
- Add parameter to control storage mode for backward compatibility

tests/unit/test_series_app.py

@@ -240,7 +240,7 @@ class TestSeriesAppReScan:
     async def test_rescan_success(
         self, mock_serie_list, mock_scanner, mock_loaders
     ):
-        """Test successful directory rescan."""
+        """Test successful directory rescan (file-based mode)."""
         test_dir = "/test/anime"
         app = SeriesApp(test_dir)
         
@@ -252,8 +252,8 @@ class TestSeriesAppReScan:
         app.serie_scanner.reinit = Mock()
         app.serie_scanner.scan = Mock()
         
-        # Perform rescan
-        await app.rescan()
+        # Perform rescan with file-based mode (use_database=False)
+        await app.rescan(use_database=False)
         
         # Verify rescan completed
         app.serie_scanner.reinit.assert_called_once()
@@ -266,7 +266,7 @@ class TestSeriesAppReScan:
     async def test_rescan_with_callback(
         self, mock_serie_list, mock_scanner, mock_loaders
     ):
-        """Test rescan with progress callbacks."""
+        """Test rescan with progress callbacks (file-based mode)."""
         test_dir = "/test/anime"
         app = SeriesApp(test_dir)
         
@@ -284,8 +284,8 @@ class TestSeriesAppReScan:
         
         app.serie_scanner.scan = Mock(side_effect=mock_scan)
         
-        # Perform rescan
-        await app.rescan()
+        # Perform rescan with file-based mode (use_database=False)
+        await app.rescan(use_database=False)
         
         # Verify rescan completed
         app.serie_scanner.scan.assert_called_once()
@@ -297,7 +297,7 @@ class TestSeriesAppReScan:
     async def test_rescan_cancellation(
         self, mock_serie_list, mock_scanner, mock_loaders
     ):
-        """Test rescan cancellation."""
+        """Test rescan cancellation (file-based mode)."""
         test_dir = "/test/anime"
         app = SeriesApp(test_dir)
         
@@ -313,9 +313,9 @@ class TestSeriesAppReScan:
         
         app.serie_scanner.scan = Mock(side_effect=mock_scan)
         
-        # Perform rescan - should handle cancellation
+        # Perform rescan - should handle cancellation (file-based mode)
         try:
-            await app.rescan()
+            await app.rescan(use_database=False)
         except Exception:
             pass  # Cancellation is expected