Add reservation flow: BilletBuyer, guest checkout, Stripe payment

- Create BilletBuyer entity: event, user (nullable for guests), firstName,
  lastName, email, reference (ETICKET-XXXX-XXXX-XXXX), totalHT, status,
  stripeSessionId, paidAt, items (OneToMany)
- Create BilletBuyerItem entity: billet, billetName (snapshot), quantity,
  unitPriceHT, line total helpers
- OrderController with full checkout flow:
  - POST /evenement/{id}/commander: create order from cart JSON
  - GET/POST /commande/{id}/informations: guest form (name, email)
  - GET /commande/{id}/paiement: payment page with recap
  - POST /commande/{id}/stripe: Stripe Checkout on connected account
    with application_fee, productId, and quantities
  - GET /commande/{id}/confirmation: success page
- Cart JS: POST cart data on Commander click, redirect to guest/payment
- Templates: guest form, payment page, order summary partial, success page
- Stripe payment uses organizer connected account, application_fee based
  on commissionRate, existing productId when available
- Tests: BilletBuyerTest (12), BilletBuyerItemTest (6), cart.test.js (13)

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

assets/modules/cart.js

@@ -60,5 +60,41 @@ export function initCart() {
         })
     }
 
+    if (checkoutBtn) {
+        checkoutBtn.addEventListener('click', () => {
+            const cart = []
+            for (const item of items) {
+                const qty = Number.parseInt(item.querySelector('[data-cart-qty]').value, 10) || 0
+                if (qty > 0) {
+                    cart.push({ billetId: item.dataset.billetId, qty })
+                }
+            }
+
+            if (cart.length === 0) return
+
+            const orderUrl = checkoutBtn.dataset.orderUrl
+            if (!orderUrl) return
+
+            checkoutBtn.disabled = true
+            checkoutBtn.textContent = 'Chargement...'
+
+            globalThis.fetch(orderUrl, {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify(cart),
+            })
+                .then(r => r.json())
+                .then(data => {
+                    if (data.redirect) {
+                        globalThis.location.href = data.redirect
+                    }
+                })
+                .catch(() => {
+                    checkoutBtn.disabled = false
+                    checkoutBtn.textContent = 'Commander'
+                })
+        })
+    }
+
     updateTotals()
 }