Move repository interfaces to Application layer

Refactored IRepository<T> and ICatalogRepository to reside in the DbFirst.Application layer instead of Domain. Updated namespaces, using statements, and all references in services and handlers. Adjusted csproj dependencies to reflect the new structure. Updated comments to clarify Clean Architecture rationale and improved separation of concerns.

DbFirst.API/Program.cs

@@ -1,6 +1,6 @@
 using DbFirst.API.Middleware;
 using DbFirst.Application;
-using DbFirst.Domain.Repositories;
+using DbFirst.Application.Repositories;
 using DbFirst.Infrastructure;
 using DbFirst.Infrastructure.Repositories;