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/Tracks/GetTrackById.cs

@@ -0,0 +1,28 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks
+{
+    public record GetTrackByIdQuery(Guid TrackId)
+        : IQuery<ErrorOr<TrackApplicationResponse>>;
+
+    public class GetTrackByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetTrackByIdQuery, ErrorOr<TrackApplicationResponse> >
+    {
+        public async ValueTask<ErrorOr<TrackApplicationResponse>> Handle(GetTrackByIdQuery request, CancellationToken cancellationToken)
+        {
+            var track = await database.Tracks.AsNoTracking()
+                .Where(t => t.Id == request.TrackId)
+                .Select(t => TrackApplicationResponse.FromEntity(t))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (track is null)
+                return Error.NotFound();
+
+            return track;
+        }
+    }
+}