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/RoutingSlip/Builders/AudioWorkflowRoutingSlipBuilder.cs

@@ -60,6 +60,12 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                     message.TrackId,
                     message.DestinationFolderKey));
 
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.TrackId, message.TrackId);
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.ProcessKind, RoutingSlipVariableNames.ProcessKinds.TrackAudio);
+            routingSlipBuilder.AddSubscription(
+                EndpointHelper.BuildConsumerUri(TrackProcessingSlipFaultConsumer.QueueName),
+                RoutingSlipEvents.Faulted);
+
             return routingSlipBuilder;
         }
     }

backend/Musify.Infrastructure/MassTransit/RoutingSlip/Builders/PictureWorkflowRoutingSlipBuilder.cs

@@ -32,6 +32,12 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                     RoutingSlipVariableNames.Picture.MediumResizedFilePath,
                     RoutingSlipVariableNames.Picture.LargeResizedFilePath));
 
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.TrackId, message.TrackId);
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.ProcessKind, RoutingSlipVariableNames.ProcessKinds.TrackPicture);
+            routingSlipBuilder.AddSubscription(
+                EndpointHelper.BuildConsumerUri(TrackProcessingSlipFaultConsumer.QueueName),
+                RoutingSlipEvents.Faulted);
+
             return routingSlipBuilder;
         }