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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Application/Users/Commands/CreateUserCommand.cs

@@ -0,0 +1,26 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Users.Responses;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users.Commands
+{
+    public record CreateUserCommand(long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName)
+        : ICommand<Result<UserApplicationResponse>>
+    {
+        public static User ToEntity(CreateUserCommand command)
+        {
+            return new User
+            {
+                Id = command.Id,
+                Name = command.Name,
+                NormalizedName = command.Name.ToUpperInvariant(),
+                FirstName = command.FirstName,
+                SecondName = command.SecondName
+            };
+        }
+    }
+}