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/MassTransitEventBus.cs

@@ -0,0 +1,16 @@
+using MassTransit;
+using Musify.Application.Contracts;
+
+namespace Musify.Infrastructure.MassTransit
+{
+    public class MassTransitEventBus(IPublishEndpoint publishEndpoint)
+        : IEventBus
+    {
+        public async Task PublishAsync<T>(T message, CancellationToken cancellationToken) where T : class
+        {
+            ArgumentNullException.ThrowIfNull(message, nameof(message));
+
+            await publishEndpoint.Publish(message, cancellationToken);
+        }
+    }
+}