feat: integrate Stripe payment with bugfixes and active timeout cancellation

- Add Stripe payment provider with Checkout Session flow
- Payment provider abstraction layer (EasyPay + Stripe unified interface)
- Stripe webhook with proper raw body handling and signature verification
- Frontend: Stripe button with URL validation, anti-duplicate click, noopener
- Active timeout cancellation: query platform before expiring, recover paid orders
- Singleton Stripe client, idempotency keys, Math.round for amounts
- Handle async_payment events, return null for unknown webhook events
- Set Checkout Session expires_at aligned with order timeout
- Add cancelPayment to provider interface (Stripe: sessions.expire, EasyPay: no-op)
- Enable stripe in frontend payment type list

src/lib/payment/index.ts

@@ -0,0 +1,30 @@
+import { paymentRegistry } from './registry';
+import { EasyPayProvider } from '@/lib/easy-pay/provider';
+import { StripeProvider } from '@/lib/stripe/provider';
+import { getEnv } from '@/lib/config';
+
+export { paymentRegistry } from './registry';
+export type {
+  PaymentType,
+  PaymentProvider,
+  CreatePaymentRequest,
+  CreatePaymentResponse,
+  QueryOrderResponse,
+  PaymentNotification,
+  RefundRequest,
+  RefundResponse,
+} from './types';
+
+let initialized = false;
+
+export function initPaymentProviders(): void {
+  if (initialized) return;
+  paymentRegistry.register(new EasyPayProvider());
+
+  const env = getEnv();
+  if (env.STRIPE_SECRET_KEY) {
+    paymentRegistry.register(new StripeProvider());
+  }
+
+  initialized = true;
+}