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/Database.cs

@@ -3,6 +3,7 @@ using Microsoft.EntityFrameworkCore.Storage;
 using MassTransit;
 using Musify.Domain.Entities;
 using Musify.Infrastructure.Configuration;
+using Musify.Infrastructure.MassTransit.Sagas;
 using AppIDatabase = Musify.Application.Contracts.IDatabase;
 using Musify.Application.Contracts;
 
@@ -24,6 +25,11 @@ namespace Musify.Infrastructure.Persistence
             modelBuilder.AddInboxStateEntity();
             modelBuilder.AddOutboxMessageEntity();
             modelBuilder.AddOutboxStateEntity();
+
+            var trackProcessing = modelBuilder.Entity<TrackProcessingState>();
+            trackProcessing.HasKey(state => state.CorrelationId);
+            trackProcessing.Property(state => state.CorrelationId).ValueGeneratedNever();
+            trackProcessing.Property(state => state.CurrentState).HasMaxLength(64);
         }
 
         public DbSet<User> Users => Set<User>();