Add BilletOrder entity, PDF generation, email with QR codes, public order page

- BilletOrder entity: individual tickets with unique ETICKET-XXXX reference,
  billetBuyer link, billet link, isScanned, scannedAt for entry control
- BilletOrderService: generates tickets after payment, creates A4 PDF with
  BilletDesign colors if present (default otherwise), real QR code via
  endroid/qr-code, event poster + org logo as base64, sends confirmation
  email with all ticket PDFs attached
- PDF template (pdf/billet.html.twig): A4 layout matching preview design,
  real QR code linking to /ticket/verify/{reference}
- Email template: order recap table, ticket references list, link to
  /ma-commande/{reference}
- Public order page /ma-commande/{reference}: no auth required, shows
  order details, ticket list with individual PDF download links
- Ticket verification page /ticket/verify/{reference}: shows valid/scanned
  status with ticket and event details
- Download route /ma-commande/{ref}/billet/{ticketRef}: generates PDF on-the-fly
- Migration for billet_order table with unique reference index
- BilletOrderTest: 8 tests, 24 assertions

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

phpstan.neon

@@ -18,3 +18,4 @@ parameters:
                 - src/Entity/BilletDesign.php
                 - src/Entity/BilletBuyer.php
                 - src/Entity/BilletBuyerItem.php
+                - src/Entity/BilletOrder.php