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/DeleteCatalogCommand.cs

@@ -0,0 +1,5 @@
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Commands;
+
+public record DeleteCatalogCommand(int Id) : IRequest<bool>;