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/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);
+        }
+    }
+}