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/GetPlayListById.cs

@@ -0,0 +1,29 @@
+using ErrorOr;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.PlayLists.Responses;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists
+{
+    public record GetPlayListByIdQuery(Guid Id)
+        : IQuery<ErrorOr<PlayListApplicationResponse>>;
+
+    public class GetPlayListByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListByIdQuery, ErrorOr<PlayListApplicationResponse>>
+    {
+        public async ValueTask<ErrorOr<PlayListApplicationResponse>> Handle(GetPlayListByIdQuery request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists
+                .AsNoTracking()
+                .Where(p => p.Id == request.Id)
+                .Select(p => PlayListApplicationResponse.FromEntity(p))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (playList is null)
+                return Error.NotFound();
+
+            return playList;
+        }
+    }
+}