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.
Cover art lookup now honors an explicit prefer_source first,
falls back to the runtime primary metadata source when unset,
and uses the shared source priority for the remaining fallbacks.