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

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

resources/views/pdf/invoice/header.blade.php

@@ -26,12 +26,12 @@
         <td class="invoice-number">
             <p>
                 Ký hiệu <span class="italic">(Serial)</span>:
-                <strong>1C26TCP</strong>
+                <strong>{{ $invoice->vat_serial ?: '1C26TCP' }}</strong>
             </p>
 
             <p>
                 Số <span class="italic">(No.)</span>:
-                <strong>{{ $invoice->invoice_number ?? '<Chưa cấp số>' }}</strong>
+                <strong>{{ $invoice->vat_number ?: $invoice->invoice_number }}</strong>
             </p>
         </td>
     </tr>