chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/Consumers/UpdateTrackPictureConsumer.cs

@@ -0,0 +1,23 @@
+using MassTransit;
+using Musify.Application.Events;
+using Musify.Infrastructure.MassTransit.RoutingSlip.Builders;
+
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    public class UpdateTrackPictureConsumer(
+        IBus bus,
+        PictureWorkflowRoutingSlipBuilder routingSlipBuilder)
+        : IConsumer<UpdateTrackPictureEvent>
+    {
+        public const string QueueName = "update-track-picture";
+
+        public async Task Consume(ConsumeContext<UpdateTrackPictureEvent> consumeContext)
+        {
+            var routingSlip = routingSlipBuilder
+                .Build(consumeContext.Message, consumeContext.CorrelationId)
+                .Build();
+
+            await bus.Execute(routingSlip);
+        }
+    }
+}