refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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);
+        }
+    }
+}