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/registry.ts

@@ -0,0 +1,29 @@
+import type { PaymentProvider, PaymentType } from './types';
+
+export class PaymentProviderRegistry {
+  private providers = new Map<PaymentType, PaymentProvider>();
+
+  register(provider: PaymentProvider): void {
+    for (const type of provider.supportedTypes) {
+      this.providers.set(type, provider);
+    }
+  }
+
+  getProvider(type: PaymentType): PaymentProvider {
+    const provider = this.providers.get(type);
+    if (!provider) {
+      throw new Error(`No payment provider registered for type: ${type}`);
+    }
+    return provider;
+  }
+
+  hasProvider(type: PaymentType): boolean {
+    return this.providers.has(type);
+  }
+
+  getSupportedTypes(): PaymentType[] {
+    return Array.from(this.providers.keys());
+  }
+}
+
+export const paymentRegistry = new PaymentProviderRegistry();