Remove legacy key file support after DB migration

- SerieScanner: Remove key file fallback, keep data file fallback
- SystemSettings: Add legacy_key_cleanup_completed flag
- initialization_service: Add cleanup task to remove key files from folders with DB entries
- Tests updated to reflect key file removal from legacy path

Key files caused duplicate key errors on folder rename. DB is now sole source of truth.

tests/unit/test_serie_scanner_db_lookup.py

@@ -77,23 +77,19 @@ class TestGetSerieFromFolderDbLookup:
         assert result.key == "rooster-fighter"
         lookup.assert_called_once_with("Rooster Fighter (2026)")
 
-    def test_legacy_key_file_as_last_resort(self, temp_directory, mock_loader):
-        """No DB, no callback -> legacy 'key' file used with deprecation warning."""
+    def test_no_db_no_callback_generates_key_from_folder_name(self, temp_directory, mock_loader):
+        """No DB entry, no callback -> key generated from folder name."""
         folder = os.path.join(temp_directory, "Legacy Series")
         os.makedirs(folder, exist_ok=True)
-        with open(os.path.join(folder, "key"), "w") as f:
-            f.write("legacy-key")
+        # No key file, no data file - should fall through to Step 4 (key generation)
 
         scanner = SerieScanner(temp_directory, mock_loader)
 
-        with patch.object(logging.getLogger("src.core.SerieScanner"), "warning") as mock_warning:
-            result = scanner._SerieScanner__read_data_from_file("Legacy Series")
+        result = scanner._SerieScanner__read_data_from_file("Legacy Series")
 
-            assert result is not None
-            assert result.key == "legacy-key"
-            mock_warning.assert_called()
-            warning_calls = [str(c) for c in mock_warning.call_args_list]
-            assert any("deprecated" in c or "v3.0.0" in c for c in warning_calls)
+        assert result is not None
+        assert result.key == "legacy-series"
+        assert result.folder == "Legacy Series"
 
     def test_db_lookup_exception_caught_and_logged(self, temp_directory, mock_loader):
         """DB exception -> fallback to provider callback."""