feat(infra): track saga failure handling + cleanup (phase 2)

On definitive failure of the picture or audio slip, the saga now reaches a
Failed state and recovers end-to-end:
- Picture/audio track slips carry TrackId + ProcessKind variables and a
  subscription to a fault translator (TrackProcessingSlipFaultConsumer) that
  turns RoutingSlipFaulted into a typed Track{Picture,Audio}ProcessingFailed
- The saga stores the final object keys from the start event and, on failure,
  transitions to Failed and publishes TrackProcessingFailed
- TrackProcessingFailedConsumer runs a cleanup slip that removes the orphaned
  final picture/audio copies and marks the track LifeCycleStatus=Failed
  (MarkTrackAsFailedActivity) — the track stays as a visible, recoverable record
- Migration for the new saga columns

Verified: Worker boots against RabbitMQ + Postgres in Docker; the new
endpoints (fault translator, failed consumer, mark-track-as-failed) configure
and the bus starts clean.

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/Activities/Tracks/MarkTrackAsFailedActivity.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.Tracks
+{
+    internal class MarkTrackAsFailedActivity(
+        IDatabase database,
+        ILogger<MarkTrackAsFailedActivity> logger)
+        : IExecuteActivity<MarkTrackAsFailedArguments>
+    {
+        public const string ExecuteEndpointName = "mark-track-as-failed";
+
+        public async Task<ExecutionResult> Execute(ExecuteContext<MarkTrackAsFailedArguments> executeContext)
+        {
+            var track = await database.Tracks
+                .SingleOrDefaultAsync(t => t.Id == executeContext.Arguments.TrackId, executeContext.CancellationToken);
+
+            if (track is not null)
+            {
+                track.LifeCycleStatus = LifeCycleStatus.Failed;
+                await database.SaveChangesAsync(executeContext.CancellationToken);
+                logger.LogInformation("Marked track {TrackId} as failed", executeContext.Arguments.TrackId);
+            }
+
+            return executeContext.Completed();
+        }
+    }
+}