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/GetAllCatalogsHandler.cs

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+using AutoMapper;
+using DbFirst.Domain.Repositories;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Queries;
+
+public class GetAllCatalogsHandler : IRequestHandler<GetAllCatalogsQuery, List<CatalogReadDto>>
+{
+    private readonly ICatalogRepository _repository;
+    private readonly IMapper _mapper;
+
+    public GetAllCatalogsHandler(ICatalogRepository repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    public async Task<List<CatalogReadDto>> Handle(GetAllCatalogsQuery request, CancellationToken cancellationToken)
+    {
+        var items = await _repository.GetAllAsync(cancellationToken);
+        return _mapper.Map<List<CatalogReadDto>>(items);
+    }
+}

DbFirst.Application/Catalogs/Queries/GetAllCatalogsQuery.cs

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+using DbFirst.Application.Catalogs;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Queries;
+
+public record GetAllCatalogsQuery : IRequest<List<CatalogReadDto>>;

DbFirst.Application/Catalogs/Queries/GetCatalogByIdHandler.cs

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+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);
+    }
+}

DbFirst.Application/Catalogs/Queries/GetCatalogByIdQuery.cs

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+using DbFirst.Application.Catalogs;
+using MediatR;
+
+namespace DbFirst.Application.Catalogs.Queries;
+
+public record GetCatalogByIdQuery(int Id) : IRequest<CatalogReadDto?>;