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/app/api/stripe/webhook/route.ts

@@ -0,0 +1,35 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { initPaymentProviders, paymentRegistry } from '@/lib/payment';
+import type { PaymentType } from '@/lib/payment';
+import { handlePaymentNotify } from '@/lib/order/service';
+
+// Stripe needs raw body - ensure Next.js doesn't parse it
+export const dynamic = 'force-dynamic';
+
+export async function POST(request: NextRequest): Promise<NextResponse> {
+  try {
+    initPaymentProviders();
+    const provider = paymentRegistry.getProvider('stripe' as PaymentType);
+
+    const rawBody = Buffer.from(await request.arrayBuffer());
+    const headers: Record<string, string> = {};
+    request.headers.forEach((value, key) => {
+      headers[key.toLowerCase()] = value;
+    });
+
+    const notification = await provider.verifyNotification(rawBody, headers);
+    if (!notification) {
+      // Unknown event type — acknowledge receipt
+      return NextResponse.json({ received: true });
+    }
+    await handlePaymentNotify(notification, provider.name);
+
+    return NextResponse.json({ received: true });
+  } catch (error) {
+    console.error('Stripe webhook error:', error);
+    return NextResponse.json(
+      { error: 'Webhook processing failed' },
+      { status: 400 },
+    );
+  }
+}