Add skill management system: implement Skill and PetSkill models, create SkillController, and update ApplicationDbContext for skills

Models/Inventory.cs

@@ -1,28 +0,0 @@
-namespace PetCompanion.Models
-{
-    public class Inventory
-    {
-        public List<Item> Items { get; set; } = new();
-        public int Capacity { get; set; }
-
-        public bool AddItem(Item item)
-        {
-            if (Items.Count < Capacity)
-            {
-                Items.Add(item);
-                return true;
-            }
-            return false;
-        }
-
-        public bool RemoveItem(Item item)
-        {
-            return Items.Remove(item);
-        }
-
-        public void TrashItem(Item item)
-        {
-            RemoveItem(item);
-        }
-    }
-}