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/Hosts/Musify.Api/AppSettings.json

@@ -49,6 +49,7 @@
         "OtlpEndpoint": ""
     },
     "YouTube": {
+        "Enabled": true,
         "GeographicalLocation": "US",
         "StreamUrlCacheSeconds": 1800,
         "SearchContinuationCacheSeconds": 600,

backend/Hosts/Musify.Api/Authentication/CurrentUser.cs

@@ -8,8 +8,7 @@ public sealed class CurrentUser : IBindableFromHttpContext<CurrentUser>
     private CurrentUser(ClaimsPrincipal principal)
     {
         Principal = principal;
-        var isAuthenticated = principal.Identity?.IsAuthenticated;
-        IsAuthenticated = isAuthenticated.HasValue && isAuthenticated.Value;
+        IsAuthenticated = principal.Identity?.IsAuthenticated ?? false;
 
         var rawId = principal.FindFirstValue(ClaimTypes.NameIdentifier);
         if (long.TryParse(rawId, out var id) && id != 0)