refactor(application): co-locate each command/query with its handler

One file per use case under the feature folder (Tracks/PlayLists/Users),
dropping the Commands/Queries/Handlers sub-folders and their namespaces.
Each file now holds the record and its handler in namespace
Musify.Application.<Feature>; external usings updated accordingly.

Build: 0 warnings, 0 errors.

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

backend/Musify.Application/Users/CreateUser.cs

@@ -0,0 +1,59 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Users.Responses;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users
+{
+    public record CreateUserCommand(long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName)
+        : ICommand<ErrorOr<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
+            };
+        }
+    }
+
+    public class CreateUserCommandHandler(IDatabase database, ILogger<CreateUserCommandHandler> logger)
+        : ICommandHandler<CreateUserCommand, ErrorOr<UserApplicationResponse> >
+    {
+        public async ValueTask<ErrorOr<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 Error.Conflict(description: $"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 Error.Failure(description: $"Failed to create user {request.Id}");
+            }
+
+            logger.LogInformation("Created user {UserId}", newUser.Id);
+            return UserApplicationResponse.FromEntity(newUser);
+        }
+    }
+}