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/Users/Handlers/CreateUserCommandHandler.cs

@@ -0,0 +1,40 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Users.Commands;
+using Musify.Application.Users.Responses;
+
+namespace Musify.Application.Users.Handlers
+{
+    public class CreateUserCommandHandler(IDatabase database, ILogger<CreateUserCommandHandler> logger)
+        : ICommandHandler<CreateUserCommand, Result<UserApplicationResponse> >
+    {
+        public async ValueTask<Result<UserApplicationResponse>> Handle(CreateUserCommand request, CancellationToken cancellationToken)
+        {
+            var userExists = await database.Users.AnyAsync(u => u.Id == request.Id, cancellationToken);
+            if (userExists)
+            {
+                logger.LogInformation("User {UserId} already exists", request.Id);
+                return Result<UserApplicationResponse>.Conflict($"User {request.Id} already exists");
+            }
+
+            var newUser = CreateUserCommand.ToEntity(request);
+
+            try
+            {
+                await database.Users.AddAsync(newUser, cancellationToken);
+                await database.SaveChangesAsync(cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to create user {UserId}", request.Id);
+                return Result.Error($"Failed to create user {request.Id}");
+            }
+
+            logger.LogInformation("Created user {UserId}", newUser.Id);
+            return Result.Created(UserApplicationResponse.FromEntity(newUser));
+        }
+    }
+}