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/Tests/Musify.Application.Tests/Mixes/GenerateMixesForUserCommandHandlerTests.cs

@@ -1,8 +1,10 @@
+using ErrorOr;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
 using Musify.Application.Mixes;
 using Musify.Application.Tests.TestSupport;
 using Musify.Domain.Entities;
+using Musify.Domain.ValueObjects;
 using NSubstitute;
 using Xunit;
 
@@ -72,4 +74,33 @@ public sealed class GenerateMixesForUserCommandHandlerTests : HandlerTestBase
         Assert.DoesNotContain(mixes, mix => mix.Title == "Stale Mix");
         Assert.Empty(await Database.MixItems.Where(item => item.MixId == staleMix.Id).ToListAsync(TestContext.Current.CancellationToken));
     }
+
+    [Fact]
+    public async Task Handle_YouTubeSourceUnavailable_StillCreatesMixesFromLocalCandidatesOnly()
+    {
+        // Simulates IYouTubeMusicService.SearchSongsAsync as it behaves when the YouTube source is
+        // disabled (DisabledYouTubeMusicService, see Infrastructure): every call fails. Mix
+        // generation must degrade to local-only candidates instead of failing the whole command.
+        youTubeMusicService
+            .SearchSongsAsync(Arg.Any<string>(), Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(Error.Forbidden(description: "The YouTube Music source is disabled."));
+
+        var user = TestEntities.User();
+        var artist = TestEntities.Artist("Seed Artist");
+        var listenedTrack = TestEntities.ExternalTrack(externalId: "listened", title: "Listened Track");
+        var otherTrackByArtist = TestEntities.ExternalTrack(externalId: "not-listened", title: "Other Track");
+
+        await SeedAsync(
+            user, artist, listenedTrack, otherTrackByArtist,
+            new TrackArtist { TrackId = listenedTrack.Id, ArtistId = artist.Id, Position = 0 },
+            new TrackArtist { TrackId = otherTrackByArtist.Id, ArtistId = artist.Id, Position = 0 },
+            TestEntities.ListeningHistory(user.Id, listenedTrack.Id));
+
+        var result = await CreateHandler().Handle(new GenerateMixesForUserCommand(user.Id), TestContext.Current.CancellationToken);
+
+        Assert.False(result.IsError);
+        var items = await Database.MixItems.ToListAsync(TestContext.Current.CancellationToken);
+        Assert.NotEmpty(items);
+        Assert.All(items, item => Assert.Equal(MixItemSource.Musify, item.Source));
+    }
 }