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.Worker/Program.cs

@@ -0,0 +1,27 @@
+using Musify.Application.Configuration;
+using Musify.Infrastructure.MassTransit;
+using Musify.Infrastructure.Persistence;
+using Musify.Infrastructure.Services;
+using Musify.Worker;
+
+var builder = Host.CreateApplicationBuilder();
+
+var configuration = builder.Configuration;
+
+var services = builder.Services;
+
+services.AddApplicationStorageConfiguration(configuration);
+services.AddPlayListConfiguration(configuration);
+services.AddTrackConfiguration(configuration);
+services.AddStorageService(configuration);
+services.AddAudioTranscoder(configuration);
+services.AddPictureService(configuration);
+services.AddDatabase(configuration);
+services.AddMassTransitConsumers(configuration);
+services.AddUploadIntentJobs(configuration);
+
+services.AddHostedService<Worker>();
+
+var host = builder.Build();
+
+host.Run();