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/Persistence/Migrations/20260705124807_AddTrackProcessingSagaKeys.cs

@@ -0,0 +1,48 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Musify.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddTrackProcessingSagaKeys : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "AudioKey",
+                table: "TrackProcessingState",
+                type: "text",
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "Bucket",
+                table: "TrackProcessingState",
+                type: "text",
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "PictureKey",
+                table: "TrackProcessingState",
+                type: "text",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "AudioKey",
+                table: "TrackProcessingState");
+
+            migrationBuilder.DropColumn(
+                name: "Bucket",
+                table: "TrackProcessingState");
+
+            migrationBuilder.DropColumn(
+                name: "PictureKey",
+                table: "TrackProcessingState");
+        }
+    }
+}