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.Application/PlayLists/Commands/RemoveTrackFromPlayListCommand.cs

@@ -0,0 +1,8 @@
+using Ardalis.Result;
+using Mediator;
+
+namespace Musify.Application.PlayLists.Commands
+{
+    public record RemoveTrackFromPlayListCommand(long UserId, Guid PlayListId, Guid TrackId)
+        : ICommand<Result>;
+}