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/Queries/GetCatalogByIdHandler.cs

@@ -0,0 +1,23 @@
+using AutoMapper;
+using DbFirst.Domain.Repositories;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Queries;
+
+public class GetCatalogByIdHandler : IRequestHandler<GetCatalogByIdQuery, CatalogReadDto?>
+{
+    private readonly ICatalogRepository _repository;
+    private readonly IMapper _mapper;
+
+    public GetCatalogByIdHandler(ICatalogRepository repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    public async Task<CatalogReadDto?> Handle(GetCatalogByIdQuery request, CancellationToken cancellationToken)
+    {
+        var item = await _repository.GetByIdAsync(request.Id, cancellationToken);
+        return item == null ? null : _mapper.Map<CatalogReadDto>(item);
+    }
+}