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/MassTransit/Sagas/TrackProcessingState.cs

@@ -0,0 +1,19 @@
+using MassTransit;
+
+namespace Musify.Infrastructure.MassTransit.Sagas
+{
+    public class TrackProcessingState : SagaStateMachineInstance
+    {
+        public Guid CorrelationId { get; set; }
+
+        public string CurrentState { get; set; } = null!;
+
+        public bool PictureProcessed { get; set; }
+
+        public bool AudioProcessed { get; set; }
+
+        public DateTime CreatedAt { get; set; }
+
+        public DateTime UpdatedAt { get; set; }
+    }
+}