The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

### Issue description
A test writes a real file to disk under `self.media.filepath.parent` and does not remove it. This leaves artifacts under the configured download directory and can make tests environment-dependent.

### Issue Context
`Media.filepath` is derived from `Source.directory_path`, and `Source.directory_path` is rooted at `media_file_storage.location`, which is configured to `settings.DOWNLOAD_ROOT`. `DOWNLOAD_ROOT` defaults to a directory under the project base directory.

### Fix Focus Areas
- tubesync/sync/tests/test_media.py[259-262]
- tubesync/sync/models/media.py[845-848]
- tubesync/sync/models/source.py[467-479]
- tubesync/sync/models/_migrations.py[6-10]
- tubesync/tubesync/settings.py[136-142]

### Suggested fix
In the test, either:
1) Use `tempfile.TemporaryDirectory()` and `@override_settings(DOWNLOAD_ROOT=Path(tmpdir))` (and recreate `media_file_storage` if needed), or
2) Register cleanup with `self.addCleanup(lambda: filepath.unlink(missing_ok=True))` and remove created directories if appropriate.

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

### Issue description
`test_directory_prefix` assigns to `settings.SOURCE_DOWNLOAD_DIRECTORY_PREFIX` and does not restore the original value. This leaks global state across the test suite and can make other tests order-dependent.

### Issue Context
`Source.type_directory_path` uses `settings.SOURCE_DOWNLOAD_DIRECTORY_PREFIX` to decide whether to prepend `audio/` or `video/` directories, so changing the setting changes `Source.directory_path` calculations globally.

### Fix Focus Areas
- tubesync/sync/tests/test_filepath.py[158-175]
- tubesync/sync/models/source.py[467-479]

### Suggested fix
Wrap the test (or each sub-assertion) with Django’s `@override_settings(SOURCE_DOWNLOAD_DIRECTORY_PREFIX=True/False)` or store the original value and restore it via `self.addCleanup(...)`/`try...finally`.