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/DependencyInjection.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
+using MediatR;
 
 namespace DbFirst.Application;
 
@@ -7,6 +8,7 @@ public static class DependencyInjection
     public static IServiceCollection AddApplication(this IServiceCollection services)
     {
         services.AddAutoMapper(typeof(DependencyInjection).Assembly);
+        services.AddMediatR(typeof(DependencyInjection).Assembly);
         return services;
     }
 }