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

docker-compose.yml

@@ -2,7 +2,7 @@ services:
   app:
     build: .
     ports:
-      - "${APP_PORT:-3001}:3000"
+      - '${APP_PORT:-3001}:3000'
     env_file: .env
     environment:
       - DATABASE_URL=postgresql://sub2apipay:${DB_PASSWORD:-password}@db:5432/sub2apipay
@@ -20,7 +20,7 @@ services:
     volumes:
       - pgdata:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U sub2apipay"]
+      test: ['CMD-SHELL', 'pg_isready -U sub2apipay']
       interval: 5s
     restart: unless-stopped