feat(infra): playlist processing saga + generalize fault handling (phase 3)

Adds the PlayList picture-processing lifecycle saga (correlated by PlayListId)
mirroring the track one, and generalizes the failure plumbing:
- Rename the track fault translator to ProcessingSlipFaultConsumer and use a
  generic Workflow.SubjectId variable + ProcessKind; it now emits typed
  Track{Picture,Audio}/PlayListPicture ProcessingFailed events for all slips
- PlayListProcessingStateMachine (+ EF-persisted PlayListProcessingState,
  migration): starts on CreatePlayListResourcesEvent, finalizes on
  PlayListPictureProcessed, and on failure marks the playlist Failed and
  removes the orphaned final picture (PlayListProcessingFailedConsumer +
  MarkPlayListAsFailedActivity)
- UpdatePlayListPictureActivity publishes PlayListPictureProcessed on success
- Picture slip carries SubjectId + PlayListPicture kind + fault subscription

Verified end-to-end in Docker (RabbitMQ + migrated Postgres): both saga
endpoints, the fault translator and cleanup consumers configure, and an
addressing cross-check confirms all 21 activity execute queues the builders
address exist (caught + fixed a kebab mismatch on mark-play-list-as-failed).

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/Consumers/PlayListProcessingFailedConsumer.cs

@@ -0,0 +1,36 @@
+using MassTransit;
+using MassTransit.Courier.Contracts;
+using Musify.Application.Events;
+using Musify.Infrastructure.MassTransit.Activities.Files;
+using Musify.Infrastructure.MassTransit.Activities.PlayLists;
+using Musify.Infrastructure.MassTransit.Arguments;
+using Musify.Infrastructure.MassTransit.RoutingSlip;
+
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    public class PlayListProcessingFailedConsumer(IBus bus) : IConsumer<PlayListProcessingFailed>
+    {
+        public async Task Consume(ConsumeContext<PlayListProcessingFailed> context)
+        {
+            var message = context.Message;
+
+            var builder = new RoutingSlipBuilder(NewId.NextGuid());
+            builder.AddSubscription(
+                EndpointHelper.BuildConsumerUri(RoutingSlipCleanUpConsumer.QueueName),
+                RoutingSlipEvents.Completed | RoutingSlipEvents.Faulted);
+
+            if (!string.IsNullOrWhiteSpace(message.Bucket) && !string.IsNullOrWhiteSpace(message.PictureKey))
+                builder.AddActivity(
+                    ActivityNames.RemovePlayListOriginalPicture,
+                    EndpointHelper.BuildExecuteActivityUri(RemoveFileFromBucketActivity.ExecuteEndpointName),
+                    new RemoveFileFromBucketArguments(message.Bucket, message.PictureKey));
+
+            builder.AddActivity(
+                ActivityNames.MarkPlayListAsFailed,
+                EndpointHelper.BuildExecuteActivityUri(MarkPlayListAsFailedActivity.ExecuteEndpointName),
+                new MarkPlayListAsFailedArguments(message.PlayListId));
+
+            await bus.Execute(builder.Build());
+        }
+    }
+}