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>

config/packages/nelmio_security.yaml

@@ -34,6 +34,7 @@ nelmio_security:
                 - 'https://static.cloudflareinsights.com'
                 - 'https://challenges.cloudflare.com'
                 - 'https://cdn.jsdelivr.net'
+                - 'https://js.stripe.com'
                 - 'unsafe-inline'
             style-src:
                 - 'self'
@@ -58,6 +59,7 @@ nelmio_security:
                 - 'https://challenges.cloudflare.com'
                 - 'https://nominatim.openstreetmap.org'
                 - 'https://cdn.jsdelivr.net'
+                - 'https://api.stripe.com'
             font-src:
                 - 'self'
                 - 'https://cdnjs.cloudflare.com'

config/services.yaml

@@ -7,6 +7,7 @@
 # Put parameters here that don't need to change on each machine where the app is deployed
 # https://symfony.com/doc/current/best_practices.html#use-parameters-for-application-configuration
 parameters:
+    stripe_pk: '%env(STRIPE_PK)%'
 
 services:
     # default configuration for services in *this* file