feat: add SPAYD QR payment code to invoice PDF

Generates QR code in SVG format using the SPAYD payment standard,
matching the PHP implementation. Contains: IBAN, amount, currency,
variable symbol, constant symbol, and invoice reference.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/routes/admin/invoices-pdf.ts

@@ -1,4 +1,5 @@
 import { FastifyInstance } from 'fastify';
+import QRCode from 'qrcode';
 import prisma from '../../config/database';
 import { requirePermission } from '../../middleware/auth';
 
@@ -289,6 +290,29 @@ export default async function invoicesPdfRoutes(fastify: FastifyInstance): Promi
     }
     const totalToPay = subtotal + totalVat;
 
+    // QR code - SPAYD payment format
+    let qrSvg = '';
+    try {
+      const spaydParts = [
+        'SPD*1.0',
+        'ACC:' + (invoice.bank_iban || '').replace(/ /g, ''),
+        'AM:' + totalToPay.toFixed(2),
+        'CC:' + currency,
+        'X-VS:' + (invoice.invoice_number || ''),
+        'X-KS:' + (invoice.constant_symbol || '0308'),
+        'MSG:' + t.title + ' ' + (invoice.invoice_number || ''),
+      ];
+      const spaydString = spaydParts.join('*');
+      qrSvg = await QRCode.toString(spaydString, {
+        type: 'svg',
+        errorCorrectionLevel: 'M',
+        margin: 1,
+        width: 200,
+      });
+    } catch {
+      // QR generation failed — leave empty
+    }
+
     // VAT recapitulation (always in CZK)
     const isForeign = currency.toUpperCase() !== 'CZK';
     const cnbRate = 1.0; // Skip CNB rate conversion
@@ -911,7 +935,7 @@ ${indentCSS}
   <!-- DPH rekapitulace + QR -->
   <div class="recap-section">
     <div class="qr">
-      <!-- QR code placeholder (requires QR library) -->
+      ${qrSvg}
     </div>
     <table>
       <thead>