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>

src/Plugin.php

@@ -13,6 +13,7 @@ use Unsupervised\Schedular\Payment\BillingMethodResolver;
 use Unsupervised\Schedular\Payment\PaymentRepository;
 use Unsupervised\Schedular\Payment\PaymentService;
 use Unsupervised\Schedular\Payment\ReceiptMailer;
+use Unsupervised\Schedular\Payment\StripeGateway;
 use Unsupervised\Schedular\Payment\StudioSettings;
 use Unsupervised\Schedular\Policy\AcceptanceRepository;
 use Unsupervised\Schedular\Policy\PolicyRepository;
@@ -44,7 +45,8 @@ class Plugin {
 		$paymentRepo    = new PaymentRepository( $wpdb );
 		$settings       = new StudioSettings();
 		$resolver       = new BillingMethodResolver( $settings );
-		$paymentService = new PaymentService( $paymentRepo, $resolver, new ReceiptMailer(), $bookings, $enrollments, $settings );
+		$stripe         = new StripeGateway( $settings );
+		$paymentService = new PaymentService( $paymentRepo, $resolver, new ReceiptMailer(), $bookings, $enrollments, $settings, $stripe );
 
 		( new RoleManager() )->register();
 		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites, $enrollments, $settings, $paymentRepo, $paymentService, $resolver ) )->register();