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/SyncUser.cs

@@ -0,0 +1,57 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users
+{
+    public record SyncUserCommand(
+        long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName,
+        string? ProfilePictureUrl)
+        : ICommand<ErrorOr<Success>>;
+
+    public class SyncUserCommandHandler(IDatabase database, ILogger<SyncUserCommandHandler> logger)
+        : ICommandHandler<SyncUserCommand, ErrorOr<Success>>
+    {
+        public async ValueTask<ErrorOr<Success>> Handle(SyncUserCommand request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users
+                .SingleOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
+
+            if (user is null)
+            {
+                await database.Users.AddAsync(new User
+                {
+                    Id = request.Id,
+                    Name = request.Name,
+                    NormalizedName = request.Name.ToUpperInvariant(),
+                    FirstName = request.FirstName,
+                    SecondName = request.SecondName,
+                    ProfilePictureUrl = request.ProfilePictureUrl
+                }, cancellationToken);
+
+                await database.SaveChangesAsync(cancellationToken);
+                logger.LogInformation("Provisioned user {UserId}", request.Id);
+
+                return new Success();
+            }
+
+            user.Name = request.Name;
+            user.NormalizedName = request.Name.ToUpperInvariant();
+            user.FirstName = request.FirstName;
+            user.SecondName = request.SecondName;
+            user.ProfilePictureUrl = request.ProfilePictureUrl;
+
+            var written = await database.SaveChangesAsync(cancellationToken);
+            if (written > 0)
+                logger.LogInformation("Synced profile for user {UserId}", request.Id);
+
+            return new Success();
+        }
+    }
+}