feat(billing): renewals, coupons, VAT numbering, billing cycles and invoice payment tracking

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/Http/Resources/Api/Admin/InvoiceResource.php

@@ -2,6 +2,7 @@
 
 namespace App\Http\Resources\Api\Admin;
 
+use App\Support\Billing\BillingDuplicateDetector;
 use Illuminate\Http\Request;
 use Illuminate\Http\Resources\Json\JsonResource;
 
@@ -12,6 +13,7 @@ class InvoiceResource extends JsonResource
         return [
             'id'              => $this->id,
             'invoice_number'  => $this->invoice_number,
+            'duplicate'       => BillingDuplicateDetector::forInvoice($this->id),
             'order_num'       => $this->whenLoaded('order', fn() => $this->order?->order_num),
             'order_items'     => $this->when(
                 $this->relationLoaded('order') && $this->order?->relationLoaded('orderItems'),
@@ -25,6 +27,7 @@ class InvoiceResource extends JsonResource
             ]),
             'sub_total'       => $this->sub_total,
             'discount_amount' => $this->discount_amount,
+            'coupon_code'     => $this->coupon_code,
             'tax'             => $this->tax,
             'total'           => $this->total,
             // Công nợ: đã thu bao nhiêu / còn thiếu bao nhiêu. Kế toán cần thấy ngay trên