Clean up and organize using statements across files

Removed unused and redundant using/import statements from multiple files, including command, repository, and Program.cs. No functional changes; this commit improves code clarity and organization.

DbFirst.API/Program.cs

@@ -1,10 +1,8 @@
+using DbFirst.API.Middleware;
 using DbFirst.Application;
-using DbFirst.Application.Catalogs;
 using DbFirst.Domain.Repositories;
 using DbFirst.Infrastructure;
 using DbFirst.Infrastructure.Repositories;
-using MediatR;
-using DbFirst.API.Middleware;
 
 var builder = WebApplication.CreateBuilder(args);