feat: enhance checkout process with cart integration and stock confirmation

app/Domains/Purchase/Models/Purchase.php

@@ -3,14 +3,17 @@
 namespace App\Domains\Purchase\Models;
 
 use App\Domains\Auth\Models\User;
+use App\Domains\Cart\Models\Cart;
 use App\Domains\Tenant\Models\Tenant;
 use Illuminate\Database\Eloquent\Attributes\Fillable;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Support\Collection;
 
 #[Fillable([
+    'cart_id',
     'tenant_codigo',
     'user_id',
     'status',
@@ -30,6 +33,7 @@ class Purchase extends Model
     protected function casts(): array
     {
         return [
+            'cart_id' => 'integer',
             'user_id' => 'integer',
         ];
     }
@@ -50,6 +54,14 @@ class Purchase extends Model
         return $this->belongsTo(User::class);
     }
 
+    /**
+     * @return BelongsTo<Cart, $this>
+     */
+    public function cart(): BelongsTo
+    {
+        return $this->belongsTo(Cart::class, 'cart_id')->withTrashed();
+    }
+
     /**
      * @return HasMany<PurchaseItem, $this>
      */
@@ -76,7 +88,25 @@ class Purchase extends Model
 
     public function getTotalAmount(): float
     {
-        return (float) $this->items()->sum('total');
+        if ($this->items()->exists()) {
+            return (float) $this->items()->sum('total');
+        }
+
+        $cart = $this->relationLoaded('cart') ? $this->getRelation('cart') : $this->cart;
+
+        if ($cart === null) {
+            return 0.0;
+        }
+
+        /** @var Collection<int, mixed> $items */
+        $items = $cart->relationLoaded('items')
+            ? $cart->getRelation('items')
+            : $cart->items()->with('variant.product')->get();
+
+        return $items->reduce(
+            static fn (float $carry, $item): float => $carry + ((float) ($item->variant?->product?->precio ?? 0) * (int) $item->cantidad),
+            0.0,
+        );
     }
 
     public function finalize(): void