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/GetUserById.cs

@@ -0,0 +1,29 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Users.Responses;
+
+namespace Musify.Application.Users
+{
+    public record GetUserByIdQuery(long Id)
+        : IQuery<ErrorOr<UserApplicationResponse> >;
+
+    public class GetUserByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserByIdQuery, ErrorOr<UserApplicationResponse> >
+    {
+        public async ValueTask<ErrorOr<UserApplicationResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users
+                .AsNoTracking()
+                .Where(u => u.Id == request.Id)
+                .Select(u => UserApplicationResponse.FromEntity(u))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (user is null)
+                return Error.NotFound();
+
+            return user;
+        }
+    }
+}