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/PlayLists/GetPlayListTracks.cs

@@ -0,0 +1,43 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Pagination;
+using Musify.Application.Tracks.Responses;
+using X.PagedList.EF;
+
+namespace Musify.Application.PlayLists
+{
+    public record GetPlayListTracksQuery(
+        Guid PlayListId,
+        int PageNumber = 1,
+        int PageSize = 10)
+        : IQuery<ErrorOr<PaginatedResponse<TrackApplicationResponse>>>;
+
+    public class GetPlayListTracksQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListTracksQuery, ErrorOr<PaginatedResponse<TrackApplicationResponse>> >
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<TrackApplicationResponse> >> Handle(GetPlayListTracksQuery request, CancellationToken cancellationToken)
+        {
+            var playListExists = await database.PlayLists
+                .AsNoTracking()
+                .AnyAsync(p => p.Id == request.PlayListId, cancellationToken);
+            if (!playListExists)
+                return Error.NotFound();
+
+            var tracksQuery = database.PlayListHasTracks
+                .AsNoTracking()
+                .Include(plt => plt.Track)
+                .Where(plt => plt.PlayListId == request.PlayListId);
+
+            var totalCount = await tracksQuery.CountAsync(cancellationToken);
+
+            var pagedTracks = await tracksQuery
+                .OrderBy(plt => plt.Position)
+                .Select(plt => TrackApplicationResponse.FromEntity(plt.Track))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
+        }
+    }
+}