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.py

@@ -519,23 +519,8 @@ class TestFindMp4Files:
 class TestReadDataFromFile:
     """Test __read_data_from_file method."""
 
-    def test_reads_key_file(self, mock_loader):
-        """Should read key from 'key' file."""
-        import tempfile
-
-        with tempfile.TemporaryDirectory() as tmpdir:
-            anime_folder = os.path.join(tmpdir, "SomeAnime")
-            os.makedirs(anime_folder)
-            with open(os.path.join(anime_folder, "key"), "w") as f:
-                f.write("some-key")
-
-            scanner = SerieScanner(tmpdir, mock_loader)
-            result = scanner._SerieScanner__read_data_from_file("SomeAnime")
-            assert result is not None
-            assert result.key == "some-key"
-
     def test_reads_data_file(self, mock_loader):
-        """Should read Serie from 'data' file when no 'key' file."""
+        """Should read Serie from 'data' file when no DB entry exists."""
         import tempfile
 
         with tempfile.TemporaryDirectory() as tmpdir: