Replace Stripe Checkout with Stripe Elements for in-page payment

- PaymentIntent instead of Checkout Session on connected account
- Stripe Elements Payment Element with neo-brutalist theme
- stripe-payment.js module with waitForStripe() for deferred loading
- No inline scripts (CSP compliant), data attributes on container
- Add order_number (YYYY-MM-DD-increment) to BilletBuyer
- Payment page redesign: full-width vertical layout with event info,
  buyer info, billet listing with images/descriptions, payment form
- CSP: add js.stripe.com to script-src, api.stripe.com to connect-src
- Add stripe_pk parameter in services.yaml
- Add head block to base.html.twig for page-specific scripts

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

sonar-project.properties

@@ -1,7 +1,7 @@
 sonar.projectKey=e-ticket
 sonar.projectName=E-Ticket
 sonar.sources=src,assets,templates,docker
-sonar.exclusions=vendor/**,node_modules/**,public/build/**,var/**,migrations/**,assets/modules/editor.js,assets/modules/event-map.js,assets/modules/billet-designer.js,src/Controller/StripeWebhookController.php,src/Service/StripeService.php,src/Service/PayoutPdfService.php,src/Service/BilletOrderService.php
+sonar.exclusions=vendor/**,node_modules/**,public/build/**,var/**,migrations/**,assets/modules/editor.js,assets/modules/event-map.js,assets/modules/billet-designer.js,assets/modules/stripe-payment.js,src/Controller/StripeWebhookController.php,src/Service/StripeService.php,src/Service/PayoutPdfService.php,src/Service/BilletOrderService.php
 sonar.php.version=8.4
 sonar.sourceEncoding=UTF-8
 sonar.php.coverage.reportPaths=coverage.xml