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/Persistence/Migrations/DatabaseModelSnapshot.cs

@@ -489,9 +489,15 @@ namespace Musify.Infrastructure.Persistence.Migrations
                     b.Property<Guid>("CorrelationId")
                         .HasColumnType("uuid");
 
+                    b.Property<string>("AudioKey")
+                        .HasColumnType("text");
+
                     b.Property<bool>("AudioProcessed")
                         .HasColumnType("boolean");
 
+                    b.Property<string>("Bucket")
+                        .HasColumnType("text");
+
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("timestamp with time zone");
 
@@ -500,6 +506,9 @@ namespace Musify.Infrastructure.Persistence.Migrations
                         .HasMaxLength(64)
                         .HasColumnType("character varying(64)");
 
+                    b.Property<string>("PictureKey")
+                        .HasColumnType("text");
+
                     b.Property<bool>("PictureProcessed")
                         .HasColumnType("boolean");