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

Update Items model to have relationship with cart.

app/Models/Cart.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class Cart extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'status',
+    ];
+
+    public function city(): BelongsTo
+    {
+        return $this->belongsTo(City::class);
+    }
+
+    public function company(): BelongsTo
+    {
+        return $this->belongsTo(Company::class);
+    }
+
+    public function items(): BelongsToMany
+    {
+        return $this->belongsToMany(Item::class, 'carts_items', 'cart_id', 'item_id');
+    }
+}
+