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/CreateCatalogHandler.cs

@@ -0,0 +1,36 @@
+using AutoMapper;
+using DbFirst.Domain.Entities;
+using DbFirst.Domain.Repositories;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Commands;
+
+public class CreateCatalogHandler : IRequestHandler<CreateCatalogCommand, CatalogReadDto?>
+{
+    private readonly ICatalogRepository _repository;
+    private readonly IMapper _mapper;
+
+    public CreateCatalogHandler(ICatalogRepository repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    public async Task<CatalogReadDto?> Handle(CreateCatalogCommand request, CancellationToken cancellationToken)
+    {
+        var existing = await _repository.GetByTitleAsync(request.Dto.CatTitle, cancellationToken);
+        if (existing != null)
+        {
+            return null;
+        }
+
+        var entity = _mapper.Map<VwmyCatalog>(request.Dto);
+        entity.AddedWho = "system";
+        entity.AddedWhen = DateTime.UtcNow;
+        entity.ChangedWho = "system";
+        entity.ChangedWhen = DateTime.UtcNow;
+
+        var created = await _repository.InsertAsync(entity, cancellationToken);
+        return _mapper.Map<CatalogReadDto>(created);
+    }
+}