curl -X POST "https://api.usegately.com/api/v1/plans/plan_123/subscribe" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "success_url": "https://mysite.com/success",
    "cancel_url": "https://mysite.com/pricing",
    "customer_email": "user@example.com",
    "user_id": "user_123"
  }'

{
  "success": true,
  "checkoutUrl": "https://checkout.stripe.com/c/pay/xxx",
  "sessionId": "cs_xxx"
}