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/RoutingSlip/ActivityNames.cs

@@ -6,6 +6,7 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip
         internal const string MarkTrackAsFailed = "MarkTrackAsFailed";
         internal const string DeleteTrackFromDb = "DeleteTrackFromDb";
         internal const string MarkPlayListAsRemoving = "MarkPlayListAsRemoving";
+        internal const string MarkPlayListAsFailed = "MarkPlayListAsFailed";
         internal const string DeletePlayListFromDb = "DeletePlayListFromDb";
 
         internal const string GenerateAudioWorkflowPaths = "GenerateAudioWorkflowPaths";

backend/Musify.Infrastructure/MassTransit/RoutingSlip/Builders/AudioWorkflowRoutingSlipBuilder.cs

@@ -60,10 +60,10 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                     message.TrackId,
                     message.DestinationFolderKey));
 
-            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.TrackId, message.TrackId);
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.SubjectId, message.TrackId);
             routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.ProcessKind, RoutingSlipVariableNames.ProcessKinds.TrackAudio);
             routingSlipBuilder.AddSubscription(
-                EndpointHelper.BuildConsumerUri(TrackProcessingSlipFaultConsumer.QueueName),
+                EndpointHelper.BuildConsumerUri(ProcessingSlipFaultConsumer.QueueName),
                 RoutingSlipEvents.Faulted);
 
             return routingSlipBuilder;

backend/Musify.Infrastructure/MassTransit/RoutingSlip/Builders/PictureWorkflowRoutingSlipBuilder.cs

@@ -32,10 +32,10 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                     RoutingSlipVariableNames.Picture.MediumResizedFilePath,
                     RoutingSlipVariableNames.Picture.LargeResizedFilePath));
 
-            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.TrackId, message.TrackId);
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.SubjectId, message.TrackId);
             routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.ProcessKind, RoutingSlipVariableNames.ProcessKinds.TrackPicture);
             routingSlipBuilder.AddSubscription(
-                EndpointHelper.BuildConsumerUri(TrackProcessingSlipFaultConsumer.QueueName),
+                EndpointHelper.BuildConsumerUri(ProcessingSlipFaultConsumer.QueueName),
                 RoutingSlipEvents.Faulted);
 
             return routingSlipBuilder;
@@ -61,6 +61,12 @@ namespace Musify.Infrastructure.MassTransit.RoutingSlip.Builders
                     RoutingSlipVariableNames.Picture.MediumResizedFilePath,
                     RoutingSlipVariableNames.Picture.LargeResizedFilePath));
 
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.SubjectId, message.PlayListId);
+            routingSlipBuilder.AddVariable(RoutingSlipVariableNames.Workflow.ProcessKind, RoutingSlipVariableNames.ProcessKinds.PlayListPicture);
+            routingSlipBuilder.AddSubscription(
+                EndpointHelper.BuildConsumerUri(ProcessingSlipFaultConsumer.QueueName),
+                RoutingSlipEvents.Faulted);
+
             return routingSlipBuilder;
         }
 

backend/Musify.Infrastructure/MassTransit/RoutingSlip/RoutingSlipVariableNames.cs

@@ -6,7 +6,7 @@ namespace Musify.Infrastructure.MassTransit
         {
             internal const string TemporalDirectory = "Workflow.TemporalDirectory";
             internal const string CorrelationId = "Workflow.CorrelationId";
-            internal const string TrackId = "Workflow.TrackId";
+            internal const string SubjectId = "Workflow.SubjectId";
             internal const string ProcessKind = "Workflow.ProcessKind";
         }
 
@@ -14,6 +14,7 @@ namespace Musify.Infrastructure.MassTransit
         {
             internal const string TrackPicture = "TrackPicture";
             internal const string TrackAudio = "TrackAudio";
+            internal const string PlayListPicture = "PlayListPicture";
         }
 
         internal static class Audio