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.
Move Hydrabase availability checks into metadata_service so source resolution owns the policy. Keep web_server delegating to the centralized helper and add tests for the enabled/disabled cases.
Clients are for the most part being initialized per-request, which leads to a lot of redundant client initialization, as well as noise on the logs, since each client initialization emits a row on the logs, eg. 'Deezer client initialized'
