Implement Stripe PaymentIntent service

[DYSfu]

/claim #1

Summary

• replace the timestamp payment stub with Stripe SDK PaymentIntent creation configured from STRIPE_SECRET_KEY
• validate payment payload amount, currency, and metadata before calling the provider
• return Stripe paymentIntent.id as paymentId and paymentIntent.client_secret as clientSecret
• route payment validation/provider errors through the API error handler while preserving Stripe error messages
• add mocked Stripe service/API coverage plus an opt-in live test-mode smoke check

Demo

• demos/stripe-payment-intent-demo.mp4

Validation

• npm test