Add Cart model, resource and its API, Pivot table with Items.

Update Items model to have relationship with cart.

app/Models/Item.php

@@ -5,6 +5,7 @@ namespace App\Models;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 
 class Item extends Model
 {
@@ -23,4 +24,10 @@ class Item extends Model
     {
         return $this->belongsTo(Category::class);
     }
+
+    public function carts(): BelongsToMany
+    {
+        return $this->belongsToMany(Cart::class, 'carts_items', 'item_id', 'cart_id');
+    }
 }
+