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/Tracks/GetTracks.cs

@@ -0,0 +1,31 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Pagination;
+using Musify.Application.Tracks.Responses;
+using X.PagedList.EF;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Tracks
+{
+    public record GetTracksQuery(int PageNumber = 1, int PageSize = 10)
+        : IQuery<ErrorOr<PaginatedResponse<TrackApplicationResponse> >>;
+
+    public class GetTracksQueryHandler(IDatabase database)
+        : IQueryHandler<GetTracksQuery, ErrorOr<PaginatedResponse<TrackApplicationResponse>> >
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<TrackApplicationResponse> >> Handle(GetTracksQuery request, CancellationToken cancellationToken)
+        {
+            var tracksQuery = database.UserHasTracks
+                .AsNoTracking()
+                .OrderBy(t => t.Id)
+                .Include(ut => ut.Track)
+                .Select(t => TrackApplicationResponse.FromEntity(t.Track));
+
+            var totalCount = await tracksQuery.CountAsync(cancellationToken);
+            var pagedTracks = await tracksQuery.ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
+        }
+    }
+}