Add a config toggle to enable/disable the YouTube Music source

- 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.

backend/Core/Musify.Infrastructure/Configuration/YouTubeConfiguration.cs

@@ -6,6 +6,11 @@ namespace Musify.Infrastructure.Configuration
     {
         public const string SectionName = "YouTube";
 
+        /// <summary>Turns the whole YouTube Music source on or off: search, albums, streaming
+        /// and adding new YouTube tracks all fail with a clear error while this is <c>false</c>,
+        /// and mix generation silently skips YouTube candidates instead of failing.</summary>
+        public bool Enabled { get; set; } = true;
+
         [Required]
         public string GeographicalLocation { get; set; } = "US";