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/DependencyInjection/MassTransitDependencyInjection.cs

@@ -3,6 +3,7 @@ using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Musify.Application.Contracts;
 using Musify.Infrastructure.Configuration;
+using Musify.Infrastructure.MassTransit.Sagas;
 using Musify.Infrastructure.Persistence;
 
 namespace Musify.Infrastructure.MassTransit
@@ -66,6 +67,14 @@ namespace Musify.Infrastructure.MassTransit
 
                 RegisterConsumersAndActivities(options);
 
+                options.AddSagaStateMachine<TrackProcessingStateMachine, TrackProcessingState>()
+                    .EntityFrameworkRepository(repository =>
+                    {
+                        repository.ConcurrencyMode = ConcurrencyMode.Pessimistic;
+                        repository.ExistingDbContext<Database>();
+                        repository.UsePostgres();
+                    });
+
                 options.AddConfigureEndpointsCallback((registrationContext, _, endpointConfigurator) =>
                 {
                     UseStandardRetry(endpointConfigurator);