feat(infra): track saga failure handling + cleanup (phase 2)

On definitive failure of the picture or audio slip, the saga now reaches a
Failed state and recovers end-to-end:
- Picture/audio track slips carry TrackId + ProcessKind variables and a
  subscription to a fault translator (TrackProcessingSlipFaultConsumer) that
  turns RoutingSlipFaulted into a typed Track{Picture,Audio}ProcessingFailed
- The saga stores the final object keys from the start event and, on failure,
  transitions to Failed and publishes TrackProcessingFailed
- TrackProcessingFailedConsumer runs a cleanup slip that removes the orphaned
  final picture/audio copies and marks the track LifeCycleStatus=Failed
  (MarkTrackAsFailedActivity) — the track stays as a visible, recoverable record
- Migration for the new saga columns

Verified: Worker boots against RabbitMQ + Postgres in Docker; the new
endpoints (fault translator, failed consumer, mark-track-as-failed) configure
and the bus starts clean.

Build: 0 warnings, 0 errors.

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

@@ -0,0 +1,33 @@
+using MassTransit;
+using MassTransit.Courier.Contracts;
+using Musify.Application.Events;
+
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    public class TrackProcessingSlipFaultConsumer : IConsumer<RoutingSlipFaulted>
+    {
+        public const string QueueName = "track-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.TrackId, out var trackObject)
+                || !Guid.TryParse(trackObject?.ToString(), out var trackId))
+                return;
+
+            switch (kindObject?.ToString())
+            {
+                case RoutingSlipVariableNames.ProcessKinds.TrackPicture:
+                    await context.Publish(new TrackPictureProcessingFailed(trackId));
+                    break;
+                case RoutingSlipVariableNames.ProcessKinds.TrackAudio:
+                    await context.Publish(new TrackAudioProcessingFailed(trackId));
+                    break;
+            }
+        }
+    }
+}