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/EndpointHelper.cs

@@ -0,0 +1,11 @@
+namespace Musify.Infrastructure.MassTransit.Consumers
+{
+    internal static class EndpointHelper
+    {
+        internal static Uri BuildExecuteActivityUri(string endpointName) => new($"queue:{endpointName}_execute");
+
+        internal static Uri BuildCompensateActivityUri(string endpointName) => new($"queue:{endpointName}_compensate");
+
+        internal static Uri BuildConsumerUri(string endpointName) => new($"queue:{endpointName}");
+    }
+}