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/Activities/PlayLists/MarkPlayListAsFailedActivity.cs

@@ -0,0 +1,32 @@
+using MassTransit;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Domain.ValueObjects;
+using Musify.Infrastructure.MassTransit.Arguments;
+
+namespace Musify.Infrastructure.MassTransit.Activities.PlayLists
+{
+    internal class MarkPlayListAsFailedActivity(
+        IDatabase database,
+        ILogger<MarkPlayListAsFailedActivity> logger)
+        : IExecuteActivity<MarkPlayListAsFailedArguments>
+    {
+        public const string ExecuteEndpointName = "mark-play-list-as-failed";
+
+        public async Task<ExecutionResult> Execute(ExecuteContext<MarkPlayListAsFailedArguments> executeContext)
+        {
+            var playList = await database.PlayLists
+                .SingleOrDefaultAsync(p => p.Id == executeContext.Arguments.PlayListId, executeContext.CancellationToken);
+
+            if (playList is not null)
+            {
+                playList.LifeCycleStatus = LifeCycleStatus.Failed;
+                await database.SaveChangesAsync(executeContext.CancellationToken);
+                logger.LogInformation("Marked playlist {PlayListId} as failed", executeContext.Arguments.PlayListId);
+            }
+
+            return executeContext.Completed();
+        }
+    }
+}