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>

tests/js/cart.test.js

@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { initCart } from '../../assets/modules/cart.js'
 
 function createBilletterie(billets) {
@@ -15,7 +15,7 @@ function createBilletterie(billets) {
         `
     }
 
-    html += '<span id="cart-total"></span><span id="cart-count"></span><button id="cart-checkout" disabled></button></div>'
+    html += '<span id="cart-total"></span><span id="cart-count"></span><button id="cart-checkout" disabled data-order-url="/order"></button></div>'
     document.body.innerHTML = html
 }
 
@@ -128,4 +128,70 @@ describe('initCart', () => {
         document.querySelector('[data-cart-minus]').click()
         expect(document.getElementById('cart-checkout').disabled).toBe(true)
     })
+
+    it('posts cart data on checkout click', () => {
+        const fetchMock = vi.fn().mockResolvedValue({
+            json: () => Promise.resolve({ redirect: '/commande/1/informations' }),
+        })
+        globalThis.fetch = fetchMock
+
+        createBilletterie([
+            { id: 1, price: '10.00', max: 5 },
+            { id: 2, price: '20.00', max: 3 },
+        ])
+        initCart()
+
+        const plusBtns = document.querySelectorAll('[data-cart-plus]')
+        plusBtns[0].click()
+        plusBtns[0].click()
+        plusBtns[1].click()
+
+        document.getElementById('cart-checkout').click()
+
+        expect(fetchMock).toHaveBeenCalledWith('/order', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify([
+                { billetId: '1', qty: 2 },
+                { billetId: '2', qty: 1 },
+            ]),
+        })
+    })
+
+    it('does not post when cart is empty on checkout', () => {
+        const fetchMock = vi.fn()
+        globalThis.fetch = fetchMock
+
+        createBilletterie([{ id: 1, price: '10.00', max: 5 }])
+        initCart()
+
+        document.getElementById('cart-checkout').disabled = false
+        document.getElementById('cart-checkout').click()
+
+        expect(fetchMock).not.toHaveBeenCalled()
+    })
+
+    it('does not post without order url', () => {
+        const fetchMock = vi.fn()
+        globalThis.fetch = fetchMock
+
+        document.body.innerHTML = `
+            <div id="billetterie">
+                <div data-cart-item data-billet-id="1" data-price="10" data-max="5">
+                    <button data-cart-minus></button>
+                    <input data-cart-qty type="number" min="0" max="5" value="0" readonly>
+                    <button data-cart-plus></button>
+                    <span data-cart-line-total></span>
+                </div>
+                <span id="cart-total"></span><span id="cart-count"></span>
+                <button id="cart-checkout" disabled></button>
+            </div>
+        `
+        initCart()
+
+        document.querySelector('[data-cart-plus]').click()
+        document.getElementById('cart-checkout').click()
+
+        expect(fetchMock).not.toHaveBeenCalled()
+    })
 })