Implement Stripe PaymentIntent creation

[Thanhdn1984]

Fixes #1

Summary

• add Stripe SDK dependency to the API workspace
• validate positive integer amount + 3-letter currency before creating a PaymentIntent
• require STRIPE_SECRET_KEY instead of returning a mock payment payload
• return Stripe PaymentIntent id, amount, currency, clientSecret, provider
• fix API test command glob and add payment validation/env tests

Test

• npm test -w apps/api

[Thanhdn1984]

Closing this duplicate/noisy submission to avoid wasting maintainer time. Sorry for the duplicate PRs — I will keep only the most relevant/latest submission and follow up there if needed.