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/GetUsers.cs

@@ -0,0 +1,37 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Pagination;
+using Musify.Application.Users.Responses;
+using X.PagedList.EF;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Users
+{
+    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch = null)
+        : IQuery<ErrorOr<PaginatedResponse<UserApplicationResponse> >>;
+
+    public class GetUsersQueryHandler(IDatabase database)
+        : IQueryHandler<GetUsersQuery, ErrorOr<PaginatedResponse<UserApplicationResponse> > >
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<UserApplicationResponse> >> Handle(GetUsersQuery request, CancellationToken cancellationToken)
+        {
+            var user = database.Users
+                .AsNoTracking()
+                .AsQueryable();
+
+            if (request.UsernameSearch is not null)
+            {
+                var normalizedUsername = request.UsernameSearch.Trim().ToUpperInvariant();
+                user = user.Where(u => u.NormalizedName.Contains(normalizedUsername));
+            }
+
+            var totalCount = await user.CountAsync(cancellationToken);
+            var pagedUser = await user
+                .Select(u => UserApplicationResponse.FromEntity(u))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<UserApplicationResponse>.FromPagedList(pagedUser);
+        }
+    }
+}