- Move the metadata and MusicBrainz-related tests into a dedicated tests/metadata subfolder.
- Keep the rest of the suite flat for now.
- Preserve the existing test filenames so the change stays organizational rather than behavioral.
8 test files had _DummyConfigManager missing get_active_media_server(),
causing failures when pytest ran them before the test file that had it.
Whichever file set sys.modules first won, and the incomplete dummy broke
later tests. Also fix script.js read_text() missing encoding='utf-8'
which failed on non-UTF-8 default locales.