Add live Stripe card charges (PaymentIntent + Elements + webhook)

Completes the deferred half of payments: real credit-card processing on
top of the existing ledger/e-transfer/comp foundation.

- StripeGateway wraps stripe/stripe-php: creates idempotent PaymentIntents
  (amount in cents, registration ids in metadata) and verifies webhook
  signatures. Stripe calls sit behind protected seams for unit testing.
- PaymentService::createIntent resolves the client-side step for a new
  registration (card → client secret; e-transfer → display data; comp →
  none) with caller-ownership enforcement.
- PaymentService::handleWebhook finalises a payment exactly once on
  payment_intent.succeeded (mark paid → confirm → receipt) and marks it
  failed on payment_intent.payment_failed.
- PaymentEndpoint: POST /payments/intent (book_lesson) and public,
  signature-verified POST /payments/webhook.
- PaymentRepository: setStripeIntentId / findByStripeIntentId.
- StudioSettings: us_stripe_webhook_secret option, with the webhook URL
  and required events surfaced on the settings page.
- Front end: shared payment.js mounts Stripe Payment Elements and confirms
  the card (or shows e-transfer instructions); Stripe.js enqueued only when
  configured. Wired into booking and group-class flows.

Tests: new StripeGatewayTest; PaymentService card-intent + webhook cases;
repository coverage. composer test/lint/cs all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/js/group-classes.js

@@ -142,13 +142,17 @@
                 accepted_policy_version_ids: accepted,
             }),
         })
-            .then(() => {
-                list.style.display = 'none';
-                confirm.style.display = 'block';
-            })
+            .then((res) => window.usPayment.collect('enrollment', res.id, list))
+            .then((result) => showConfirmation(window.usPayment.message(result)))
             .catch((err) => showError(err.message));
     }
 
+    function showConfirmation(message) {
+        confirm.textContent = message;
+        list.style.display = 'none';
+        confirm.style.display = 'block';
+    }
+
     function loadClasses() {
         clearError();
         list.style.display = 'block';