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/Sagas/TrackProcessingStateMachine.cs

@@ -30,6 +30,9 @@ namespace Musify.Infrastructure.MassTransit.Sagas
                     {
                         context.Saga.CreatedAt = DateTime.UtcNow;
                         context.Saga.UpdatedAt = DateTime.UtcNow;
+                        context.Saga.Bucket = context.Message.Bucket;
+                        context.Saga.PictureKey = context.Message.PictureDestinationKey;
+                        context.Saga.AudioKey = context.Message.AudioDestinationKey;
                     })
                     .TransitionTo(Processing));
 
@@ -50,10 +53,20 @@ namespace Musify.Infrastructure.MassTransit.Sagas
                     .If(context => context.Saga.PictureProcessed, binder => binder.Finalize()),
                 When(PictureFailed)
                     .Then(context => context.Saga.UpdatedAt = DateTime.UtcNow)
-                    .TransitionTo(Failed),
+                    .TransitionTo(Failed)
+                    .Publish(context => new TrackProcessingFailed(
+                        context.Saga.CorrelationId,
+                        context.Saga.Bucket,
+                        context.Saga.PictureKey,
+                        context.Saga.AudioKey)),
                 When(AudioFailed)
                     .Then(context => context.Saga.UpdatedAt = DateTime.UtcNow)
-                    .TransitionTo(Failed));
+                    .TransitionTo(Failed)
+                    .Publish(context => new TrackProcessingFailed(
+                        context.Saga.CorrelationId,
+                        context.Saga.Bucket,
+                        context.Saga.PictureKey,
+                        context.Saga.AudioKey)));
 
             During(Failed,
                 Ignore(PictureProcessed),