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.Api/OpenApi/OpenApiExtensions.cs

@@ -0,0 +1,14 @@
+using Microsoft.AspNetCore.OpenApi;
+
+namespace Musify.Api.OpenApi;
+
+public static class OpenApiExtensions
+{
+    public static IServiceCollection AddOpenApiConfiguration(this IServiceCollection services)
+    {
+        services.AddOpenApi();
+        services.ConfigureOptions<OpenApiOptionsSetup>();
+
+        return services;
+    }
+}