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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,36 @@
+using MassTransit;
+using MassTransit.Courier.Contracts;
+using Musify.Application.Events;
+
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    public class ProcessingSlipFaultConsumer : IConsumer<RoutingSlipFaulted>
+    {
+        public const string QueueName = "processing-slip-fault";
+
+        public async Task Consume(ConsumeContext<RoutingSlipFaulted> context)
+        {
+            var variables = context.Message.Variables;
+
+            if (!variables.TryGetValue(RoutingSlipVariableNames.Workflow.ProcessKind, out var kindObject))
+                return;
+
+            if (!variables.TryGetValue(RoutingSlipVariableNames.Workflow.SubjectId, out var subjectObject)
+                || !Guid.TryParse(subjectObject?.ToString(), out var subjectId))
+                return;
+
+            switch (kindObject?.ToString())
+            {
+                case RoutingSlipVariableNames.ProcessKinds.TrackPicture:
+                    await context.Publish(new TrackPictureProcessingFailed(subjectId));
+                    break;
+                case RoutingSlipVariableNames.ProcessKinds.TrackAudio:
+                    await context.Publish(new TrackAudioProcessingFailed(subjectId));
+                    break;
+                case RoutingSlipVariableNames.ProcessKinds.PlayListPicture:
+                    await context.Publish(new PlayListPictureProcessingFailed(subjectId));
+                    break;
+            }
+        }
+    }
+}