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/Services/ServicesDependencyInjection.cs

@@ -1,8 +1,10 @@
 using Amazon.S3;
 using Hangfire;
 using Hangfire.PostgreSql;
+using Microsoft.Extensions.Caching.Memory;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Musify.Application.Configuration;
 using Musify.Infrastructure.Configuration;
@@ -102,7 +104,17 @@ namespace Musify.Infrastructure.Services
             serviceDescriptors.AddValidatedOptions<YouTubeConfiguration>(configuration, YouTubeConfiguration.SectionName);
 
             serviceDescriptors.AddMemoryCache();
-            serviceDescriptors.AddSingleton<IYouTubeMusicService, YouTubeMusicService>();
+            serviceDescriptors.AddSingleton<IYouTubeMusicService>(serviceProvider =>
+            {
+                var youTubeConfiguration = serviceProvider.GetRequiredService<YouTubeConfiguration>();
+                if (!youTubeConfiguration.Enabled)
+                    return new DisabledYouTubeMusicService();
+
+                return new YouTubeMusicService(
+                    youTubeConfiguration,
+                    serviceProvider.GetRequiredService<IMemoryCache>(),
+                    serviceProvider.GetRequiredService<ILogger<YouTubeMusicService>>());
+            } );
             return serviceDescriptors;
         }