Add track publication, album types, optional track pictures and batch upload URLs

Tracks
- Track.IsPublished (migration marks existing tracks as published). New
  tracks are created unpublished.
- PUT /tracks/{id}/published and PUT /tracks/published change the flag;
  publishing requires completed audio processing (the batch endpoint
  reports skipped tracks).
- An unpublished track is only visible to its owner. A WherePublished /
  VisibleTo filter is applied to search, genres, listening history, last
  listened, likes, mixes, album and playlist contents and counts, recently
  listened albums, public user tracks, track details and stream tickets.
- isPublished is part of the track responses, GET /tracks/users/{userId}
  accepts a published filter and status-counts reports published and
  unpublished totals.
- CreateTrackRequest.pictureIntentId is optional: without it the track
  uses the album cover or the preset picture and no picture processing
  runs. CreateTrackResourcesEvent carries nullable picture fields, so
  queues must be drained before deploying.
- POST /tracks/upload-urls accepts an audios list and one optional picture
  shared by all of them, and returns an intent per file. The upload intent
  limits are raised so a batch fits.

Albums
- AlbumType (Album, EP, Single) on create, update and responses.
- CreateAlbumRequest.trackIds creates the album and links the tracks in a
  single save.

backend/Tests/Musify.Application.Tests/Tracks/DeleteTracksCommandHandlerTests.cs

@@ -11,9 +11,9 @@ namespace Musify.Application.Tests.Tracks
 {
     public sealed class DeleteTracksCommandHandlerTests : HandlerTestBase
     {
-        private readonly IEventBus eventBus = Substitute.For<IEventBus>();
+        private readonly IEventBus _eventBus = Substitute.For<IEventBus>();
 
-        private DeleteTracksCommandHandler CreateHandler() => new(Database, eventBus, NoOpLogger<DeleteTracksCommandHandler>());
+        private DeleteTracksCommandHandler CreateHandler() => new(Database, _eventBus, NoOpLogger<DeleteTracksCommandHandler>());
 
         [Fact]
         public async Task Handle_OwnTracks_MarksAllAsRemovingAndPublishesOneEventEach()
@@ -29,7 +29,7 @@ namespace Musify.Application.Tests.Tracks
             Assert.Equal(2, result.Value.DeletedTrackIds.Count);
             Assert.Empty(result.Value.FailedTrackIds);
             Assert.All(Database.Tracks.ToList(), track => Assert.Equal(LifeCycleStatus.Removing, track.LifeCycleStatus));
-            await eventBus.Received(2).PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
+            await _eventBus.Received(2).PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
         }
 
         [Fact]
@@ -45,7 +45,7 @@ namespace Musify.Application.Tests.Tracks
 
             Assert.Equal(ErrorType.Forbidden, result.FirstError.Type);
             Assert.All(Database.Tracks.ToList(), track => Assert.Equal(LifeCycleStatus.Active, track.LifeCycleStatus));
-            await eventBus.DidNotReceive().PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
+            await _eventBus.DidNotReceive().PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
         }
 
         [Fact]
@@ -75,7 +75,7 @@ namespace Musify.Application.Tests.Tracks
             var result = await CreateHandler().Handle(new DeleteTracksCommand(owner.Id, [track.Id, track.Id]), TestContext.Current.CancellationToken);
 
             Assert.Single(result.Value.DeletedTrackIds);
-            await eventBus.Received(1).PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
+            await _eventBus.Received(1).PublishAsync(Arg.Any<DeleteTrackEvent>(), Arg.Any<CancellationToken>());
         }
     }
 }