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>

migrations/Version20260321210000.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260321210000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add order_number to billet_buyer';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE billet_buyer ADD COLUMN IF NOT EXISTS order_number VARCHAR(20) DEFAULT NULL');
+        $this->addSql('CREATE UNIQUE INDEX IF NOT EXISTS UNIQ_BB_ORDER_NUMBER ON billet_buyer (order_number)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX IF EXISTS UNIQ_BB_ORDER_NUMBER');
+        $this->addSql('ALTER TABLE billet_buyer DROP COLUMN IF EXISTS order_number');
+    }
+}