feat(infra): playlist processing saga + generalize fault handling (phase 3)

Adds the PlayList picture-processing lifecycle saga (correlated by PlayListId)
mirroring the track one, and generalizes the failure plumbing:
- Rename the track fault translator to ProcessingSlipFaultConsumer and use a
  generic Workflow.SubjectId variable + ProcessKind; it now emits typed
  Track{Picture,Audio}/PlayListPicture ProcessingFailed events for all slips
- PlayListProcessingStateMachine (+ EF-persisted PlayListProcessingState,
  migration): starts on CreatePlayListResourcesEvent, finalizes on
  PlayListPictureProcessed, and on failure marks the playlist Failed and
  removes the orphaned final picture (PlayListProcessingFailedConsumer +
  MarkPlayListAsFailedActivity)
- UpdatePlayListPictureActivity publishes PlayListPictureProcessed on success
- Picture slip carries SubjectId + PlayListPicture kind + fault subscription

Verified end-to-end in Docker (RabbitMQ + migrated Postgres): both saga
endpoints, the fault translator and cleanup consumers configure, and an
addressing cross-check confirms all 21 activity execute queues the builders
address exist (caught + fixed a kebab mismatch on mark-play-list-as-failed).

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/DependencyInjection/MassTransitDependencyInjection.Registration.cs

@@ -36,8 +36,9 @@ namespace Musify.Infrastructure.MassTransit
             options.AddConsumer<CreateTrackConsumer>();
             options.AddConsumer<CreatePlayListConsumer>();
             options.AddConsumer<UpdatePlayListPictureSourceConsumer>();
-            options.AddConsumer<TrackProcessingSlipFaultConsumer>();
+            options.AddConsumer<ProcessingSlipFaultConsumer>();
             options.AddConsumer<TrackProcessingFailedConsumer>();
+            options.AddConsumer<PlayListProcessingFailedConsumer>();
 
             options.AddExecuteActivity<RemoveFileFromBucketActivity, RemoveFileFromBucketArguments>();
             options.AddExecuteActivity<GenerateAudioWorkflowPathsActivity, GenerateAudioWorkflowPathsArguments>();
@@ -49,6 +50,7 @@ namespace Musify.Infrastructure.MassTransit
             options.AddExecuteActivity<PublishTrackProcessingEventsActivity, PublishTrackProcessingEventsArguments>();
             options.AddExecuteActivity<PublishPlayListPictureProcessingEventActivity, PublishPlayListPictureProcessingEventArguments>();
             options.AddExecuteActivity<MarkTrackAsFailedActivity, MarkTrackAsFailedArguments>();
+            options.AddExecuteActivity<MarkPlayListAsFailedActivity, MarkPlayListAsFailedArguments>();
 
             options.AddActivity<ResizePictureActivity, ResizePictureLocalArguments, ResizePictureLog>();
             options.AddActivity<UpdatePlayListPictureActivity, UpdatePlayListPictureArguments, UpdatePlayListPictureLog>();

backend/Musify.Infrastructure/MassTransit/DependencyInjection/MassTransitDependencyInjection.cs

@@ -75,6 +75,14 @@ namespace Musify.Infrastructure.MassTransit
                         repository.UsePostgres();
                     });
 
+                options.AddSagaStateMachine<PlayListProcessingStateMachine, PlayListProcessingState>()
+                    .EntityFrameworkRepository(repository =>
+                    {
+                        repository.ConcurrencyMode = ConcurrencyMode.Pessimistic;
+                        repository.ExistingDbContext<Database>();
+                        repository.UsePostgres();
+                    });
+
                 options.AddConfigureEndpointsCallback((registrationContext, _, endpointConfigurator) =>
                 {
                     UseStandardRetry(endpointConfigurator);