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/Activities/Audio/UpdateTrackAudioActivity.cs

@@ -2,6 +2,7 @@ using MassTransit;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Musify.Application.Contracts;
+using Musify.Application.Events;
 using Musify.Domain.Entities;
 using Musify.Domain.ValueObjects;
 using Musify.Infrastructure.MassTransit.Arguments;
@@ -11,6 +12,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 {
     internal class UpdateTrackAudioActivity(
         IDatabase database,
+        IPublishEndpoint publishEndpoint,
         ILogger<UpdateTrackAudioActivity> logger)
         : IActivity<UpdateTrackAudioArguments, UpdateTrackAudioLog>
     {
@@ -37,6 +39,9 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
             if (string.IsNullOrWhiteSpace(audioFolderKey))
             {
                 logger.LogWarning("Audio folder key is empty for track {TrackId}; skipping update", track.Id);
+                await publishEndpoint.Publish(
+                    new TrackAudioProcessed(executeContext.Arguments.TrackId),
+                    executeContext.CancellationToken);
                 return executeContext.Completed();
             }
 
@@ -52,6 +57,10 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
                 logger.LogInformation("Updated track {TrackId} audio", executeContext.Arguments.TrackId);
 
+                await publishEndpoint.Publish(
+                    new TrackAudioProcessed(executeContext.Arguments.TrackId),
+                    executeContext.CancellationToken);
+
                 return executeContext.Completed(log);
             }
             catch (Exception exception)

backend/Musify.Infrastructure/MassTransit/Activities/Pictures/UpdateTrackPictureActivity.cs

@@ -1,6 +1,7 @@
 using MassTransit;
 using Microsoft.Extensions.Logging;
 using Musify.Application.Contracts;
+using Musify.Application.Events;
 using Musify.Domain.ValueObjects;
 using Musify.Infrastructure.MassTransit.Arguments;
 using Musify.Infrastructure.MassTransit.Logs;
@@ -9,6 +10,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Pictures
 {
     internal class UpdateTrackPictureActivity(
         IDatabase database,
+        IPublishEndpoint publishEndpoint,
         ILogger<UpdateTrackPictureActivity> logger)
         : IActivity<UpdateTrackPictureArguments, UpdateTrackPictureLog>
     {
@@ -55,6 +57,10 @@ namespace Musify.Infrastructure.MassTransit.Activities.Pictures
                 logger.LogInformation("Updated track {TrackId} pictures",
                     executeContext.Arguments.TrackId);
 
+                await publishEndpoint.Publish(
+                    new TrackPictureProcessed(executeContext.Arguments.TrackId),
+                    executeContext.CancellationToken);
+
                 return executeContext.Completed();
             }
             catch (Exception exception)