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.

backend/Musify.Application/PlayLists/GetPlayListsByUserId.cs

@@ -0,0 +1,43 @@
+using ErrorOr;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Pagination;
+using Musify.Application.PlayLists.Responses;
+using X.PagedList.EF;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists
+{
+    public record GetPlayListsByUserIdQuery(
+        long UserId,
+        string? Name,
+        int PageNumber = 1,
+        int PageSize = 10)
+        : IQuery<ErrorOr<PaginatedResponse<PlayListApplicationResponse>> >;
+
+    public class GetPlayListsByUserIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListsByUserIdQuery, ErrorOr<PaginatedResponse<PlayListApplicationResponse> > >
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<PlayListApplicationResponse> >> Handle(GetPlayListsByUserIdQuery request, CancellationToken cancellationToken)
+        {
+            var playListsQuery = database.PlayLists
+                .AsNoTracking()
+                .Where(p => p.UserId == request.UserId);
+
+            if (request.Name is not null)
+            {
+                var normalizedName = request.Name.Trim().ToUpperInvariant();
+                playListsQuery = playListsQuery.Where(p => p.NormalizedName.Contains(normalizedName));
+            }
+
+            var totalCount = await playListsQuery.CountAsync(cancellationToken);
+
+            var pagedPlayLists = await playListsQuery
+                .OrderBy(p => p.CreatedAt)
+                .Select(p => PlayListApplicationResponse.FromEntity(p))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<PlayListApplicationResponse>.FromPagedList(pagedPlayLists);
+        }
+    }
+}