- YouTubeConfiguration.Enabled (default true, Infrastructure): single
switch for the whole source
- DisabledYouTubeMusicService: null-object IYouTubeMusicService that
fails every call with Forbidden, registered instead of the real one
when disabled (AddYouTubeMusicService) so no client is ever built
- AppSettings.json (Api + Worker): document YouTube.Enabled explicitly;
overridable via YouTube__Enabled env var like every other section
- CurrentUser.cs: small unrelated cleanup, collapse the IsAuthenticated
computation to a single null-coalescing expression
Search, albums, stream resolution and adding a new YouTube track all
surface as 403 Forbidden through the existing ErrorOr -> HTTP mapping.
Daily mix generation already treated a YouTube search failure as "no
YouTube candidates" and keeps working with local-only candidates —
added a test locking that in. Already-downloaded YouTube tracks are
unaffected since playback never goes through IYouTubeMusicService.
257/257 tests passing (7 new), 0 build warnings.
