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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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/Core/Musify.Infrastructure/MassTransit/RoutingSlip/Builders/CreateTrackRoutingSlipBuilder.cs

@@ -11,11 +11,21 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
     {
         public RoutingSlipBuilder Build(CreateTrackResourcesEvent message, Guid? correlationId)
         {
-            var builder = RoutingSlips.Create(correlationId)
-                .AddStep(
+            var builder = RoutingSlips.Create(correlationId);
+
+            var hasPicture = message.PictureIntentId != null
+                && message.PictureSourceKey != null
+                && message.PictureDestinationKey != null;
+
+            if (hasPicture)
+            {
+                builder.AddStep(
                     ActivityNames.CopyPictureToFinal,
                     CopyFileInBucketActivity.ExecuteEndpointName,
-                    new CopyFileInBucketArguments(message.Bucket, message.PictureSourceKey, message.Bucket, message.PictureDestinationKey))
+                    new CopyFileInBucketArguments(message.Bucket, message.PictureSourceKey!, message.Bucket, message.PictureDestinationKey!));
+            }
+
+            builder
                 .AddStep(
                     ActivityNames.CopyAudioToFinal,
                     CopyFileInBucketActivity.ExecuteEndpointName,
@@ -23,21 +33,23 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                 .AddStep(
                     ActivityNames.ConsumeUploadIntents,
                     ConsumeUploadIntentsActivity.ExecuteEndpointName,
-                    new ConsumeUploadIntentsArguments([message.PictureIntentId, message.AudioIntentId]))
+                    new ConsumeUploadIntentsArguments(
+                        hasPicture ? [message.PictureIntentId!.Value, message.AudioIntentId] : [message.AudioIntentId]))
                 .AddStep(
                     ActivityNames.PublishTrackProcessingEvents,
                     PublishTrackProcessingEventsActivity.ExecuteEndpointName,
                     new PublishTrackProcessingEventsArguments(
                         message.TrackId,
                         message.Bucket,
-                        message.PictureDestinationKey,
+                        hasPicture ? message.PictureDestinationKey : null,
                         message.AudioDestinationKey,
                         message.AudioProcessedFolderKey,
                         message.Sizes))
                 .TrackFaults(message.TrackId, RoutingSlipVariableNames.ProcessKinds.TrackCreation);
 
             builder.AddVariable(RoutingSlipVariableNames.Workflow.Bucket, message.Bucket);
-            builder.AddVariable(RoutingSlipVariableNames.Workflow.PictureKey, message.PictureDestinationKey);
+            if (hasPicture)
+                builder.AddVariable(RoutingSlipVariableNames.Workflow.PictureKey, message.PictureDestinationKey!);
             builder.AddVariable(RoutingSlipVariableNames.Workflow.AudioKey, message.AudioDestinationKey);
 
             return builder;

backend/Core/Musify.Infrastructure/MassTransit/RoutingSlip/Builders/DeleteRoutingSlipBuilder.cs

@@ -47,7 +47,7 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                         routes.BuildOriginalPicturePath(userId, track.Pictures.OriginalName));
                 }
 
-                if (track.Pictures.IsProcessed)
+                if (track.Pictures.IsProcessed && !string.IsNullOrEmpty(track.Pictures.OriginalName))
                 {
                     builder.AddRemoveFile(ActivityNames.RemoveTrackSmallPicture, bucket, routes.BuildPicturePath(PictureSize.Small, track.Pictures.SmallName));
                     builder.AddRemoveFile(ActivityNames.RemoveTrackMediumPicture, bucket, routes.BuildPicturePath(PictureSize.Medium, track.Pictures.MediumName));