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.Infrastructure.Tests/Services/DisabledYouTubeMusicServiceTests.cs

@@ -0,0 +1,58 @@
+using ErrorOr;
+using Musify.Infrastructure.Services;
+using Xunit;
+
+namespace Musify.Infrastructure.Tests.Services;
+
+public sealed class DisabledYouTubeMusicServiceTests
+{
+    private readonly DisabledYouTubeMusicService service = new();
+
+    [Fact]
+    public async Task SearchSongsAsync_ReturnsForbidden()
+    {
+        var result = await service.SearchSongsAsync("query", string.Empty, TestContext.Current.CancellationToken);
+
+        Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
+    }
+
+    [Fact]
+    public async Task SearchAlbumsAsync_ReturnsForbidden()
+    {
+        var result = await service.SearchAlbumsAsync("query", string.Empty, TestContext.Current.CancellationToken);
+
+        Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
+    }
+
+    [Fact]
+    public async Task GetAlbumAsync_ReturnsForbidden()
+    {
+        var result = await service.GetAlbumAsync("album-1", TestContext.Current.CancellationToken);
+
+        Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
+    }
+
+    [Fact]
+    public async Task GetAudioStreamAsync_ReturnsForbidden()
+    {
+        var result = await service.GetAudioStreamAsync("video-1", TestContext.Current.CancellationToken);
+
+        Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
+    }
+
+    [Fact]
+    public async Task GetSongAsync_ReturnsForbidden()
+    {
+        var result = await service.GetSongAsync("video-1", TestContext.Current.CancellationToken);
+
+        Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
+    }
+
+    [Fact]
+    public void ResolveArtworkUrl_ReturnsTheUrlUnchanged()
+    {
+        var result = service.ResolveArtworkUrl("https://img.example/thumb.jpg");
+
+        Assert.Equal("https://img.example/thumb.jpg", result);
+    }
+}