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/DeleteTrack.cs

@@ -0,0 +1,74 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Domain.ValueObjects;
+using Musify.Application.Events;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Tracks
+{
+    public record DeleteTrackCommand(
+        long UserId,
+        Guid TrackId)
+        : ICommand<ErrorOr<Success>>;
+
+    public class DeleteTrackCommandHandler(
+        IDatabase database,
+        IEventBus eventBus,
+        ILogger<DeleteTrackCommandHandler> logger)
+        : ICommandHandler<DeleteTrackCommand, ErrorOr<Success>>
+    {
+        public async ValueTask<ErrorOr<Success>> Handle(DeleteTrackCommand request, CancellationToken cancellationToken)
+        {
+            var track = await database.Tracks.SingleOrDefaultAsync(t => t.Id == request.TrackId, cancellationToken);
+            if (track is null)
+            {
+                logger.LogInformation("Track {TrackId} not found", request.TrackId);
+                return Error.NotFound();
+            }
+
+            var isOwner = await database.UserHasTracks
+                .AsNoTracking()
+                .AnyAsync(ut => ut.TrackId == request.TrackId && ut.UserId == request.UserId, cancellationToken);
+            if (!isOwner)
+            {
+                logger.LogWarning("User {UserId} unauthorized to delete track {TrackId}", request.UserId, request.TrackId);
+                return Error.Unauthorized();
+            }
+
+            if (track.AudioTranscodeProcessingStatus == ProcessingStatus.Processing
+                || track.AudioTranscodeProcessingStatus == ProcessingStatus.Pending)
+            {
+                logger.LogWarning("Track {TrackId} is currently being processed and cannot be deleted", request.TrackId);
+                return Error.Conflict(description: "Track is currently being processed and cannot be deleted");
+            }
+
+            track.LifeCycleStatus = LifeCycleStatus.Removing;
+
+            try
+            {
+                await eventBus.PublishAsync(
+                    new DeleteTrackEvent(track.Id, request.UserId),
+                    cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to publish delete track event for track {TrackId}", request.TrackId);
+                return Error.Failure(description: $"Failed to delete track {request.TrackId}");
+            }
+
+            try
+            {
+                await database.SaveChangesAsync(cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to mark track {TrackId} as processing before deletion", request.TrackId);
+                return Error.Failure(description: $"Failed to delete track {request.TrackId}");
+            }
+
+            return new Success();
+        }
+    }
+}