Remove Upsert functionality from Catalog API

Removed Upsert endpoint and related service/repository methods. Deleted ExecuteUpsertAsync helper. Insert now throws NotImplementedException as a placeholder. Refactored Delete method in repository; logic unchanged.

DbFirst.Application/Catalogs/ICatalogService.cs

@@ -6,6 +6,5 @@ public interface ICatalogService
     Task<CatalogReadDto?> GetByIdAsync(int id, CancellationToken cancellationToken = default);
     Task<CatalogReadDto> CreateAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default);
     Task<CatalogReadDto?> UpdateAsync(int id, CatalogWriteDto dto, CancellationToken cancellationToken = default);
-    Task<CatalogReadDto?> UpsertAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default);
     Task<bool> DeleteAsync(int id, CancellationToken cancellationToken = default);
 }