Refactor namespaces to follow new naming convention and add item model classes

Data/ApplicationDbContext.cs

@@ -1,7 +1,7 @@
 using Microsoft.EntityFrameworkCore;
-using pet_companion_api.Models;
+using Pet.Companion.Models;
 
-namespace pet_companion_api.Data
+namespace Pet.Companion.Data
 {
     public class ApplicationDbContext : DbContext
     {