feat(infra): add Track processing saga (state machine, phase 1)

Introduce a MassTransit saga state machine that correlates by TrackId and
tracks the two parallel processing slips (picture + audio):
- TrackProcessingStateMachine + EF-persisted TrackProcessingState
  (pessimistic concurrency, EntityFrameworkRepository), migration added
- Starts on CreateTrackResourcesEvent; UpdateTrack{Picture,Audio}Activity now
  publish TrackPictureProcessed / TrackAudioProcessed on success; when both
  arrive the saga finalizes (Ready). Failure transitions to a Failed state
  (cleanup wired in the next phase)
- Add LifeCycleStatus.Failed and the four outcome events

Verified: booted the Worker against RabbitMQ + migrated Postgres in Docker;
the saga endpoint 'track-processing-state' is configured and the bus starts
clean.

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/Persistence/Migrations/DatabaseModelSnapshot.cs

@@ -484,6 +484,33 @@ namespace Musify.Infrastructure.Persistence.Migrations
                     b.ToTable("UserHasTrack");
                 });
 
+            modelBuilder.Entity("Musify.Infrastructure.MassTransit.Sagas.TrackProcessingState", b =>
+                {
+                    b.Property<Guid>("CorrelationId")
+                        .HasColumnType("uuid");
+
+                    b.Property<bool>("AudioProcessed")
+                        .HasColumnType("boolean");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("CurrentState")
+                        .IsRequired()
+                        .HasMaxLength(64)
+                        .HasColumnType("character varying(64)");
+
+                    b.Property<bool>("PictureProcessed")
+                        .HasColumnType("boolean");
+
+                    b.Property<DateTime>("UpdatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.HasKey("CorrelationId");
+
+                    b.ToTable("TrackProcessingState");
+                });
+
             modelBuilder.Entity("MassTransit.EntityFrameworkCoreIntegration.OutboxMessage", b =>
                 {
                     b.HasOne("MassTransit.EntityFrameworkCoreIntegration.OutboxState", null)