Add TODOs for middleware, generic service, and CQRS

Added TODO comments in Program.cs for exception handling middleware, and in CatalogService.cs and ICatalogService.cs for creating a generic service and implementing CQRS with MediatR. No functional changes made.

DbFirst.API/Program.cs

@@ -4,6 +4,8 @@ using DbFirst.Infrastructure;
 using DbFirst.Infrastructure.Repositories;
 using Microsoft.EntityFrameworkCore;
 
+//TODO: create and add exception handling middleware
+
 var builder = WebApplication.CreateBuilder(args);
 
 // Add services to the container.