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.

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

backend/Musify.Infrastructure/MassTransit/RoutingSlip/RoutingSlipVariableNames.cs

@@ -6,6 +6,14 @@ namespace Musify.Infrastructure.MassTransit
         {
             internal const string TemporalDirectory = "Workflow.TemporalDirectory";
             internal const string CorrelationId = "Workflow.CorrelationId";
+            internal const string TrackId = "Workflow.TrackId";
+            internal const string ProcessKind = "Workflow.ProcessKind";
+        }
+
+        internal static class ProcessKinds
+        {
+            internal const string TrackPicture = "TrackPicture";
+            internal const string TrackAudio = "TrackAudio";
         }
 
         internal static class Audio