Refactor Catalogs to use MediatR and CQRS pattern

Replaced direct service usage in CatalogsController with MediatR-based commands and queries for all CRUD operations. Added command/query and handler classes for Catalog operations. Updated dependency injection to register MediatR and removed ICatalogService. Improved code maintainability and testability by adopting CQRS architecture.

DbFirst.Application/Catalogs/Commands/DeleteCatalogHandler.cs

@@ -0,0 +1,19 @@
+using DbFirst.Domain.Repositories;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Commands;
+
+public class DeleteCatalogHandler : IRequestHandler<DeleteCatalogCommand, bool>
+{
+    private readonly ICatalogRepository _repository;
+
+    public DeleteCatalogHandler(ICatalogRepository repository)
+    {
+        _repository = repository;
+    }
+
+    public async Task<bool> Handle(DeleteCatalogCommand request, CancellationToken cancellationToken)
+    {
+        return await _repository.DeleteAsync(request.Id, cancellationToken);
+    }
+}