Implement Stripe payment intents

[carpedkm]

Closes #1.

/claim #1

Summary

• replaces the stub payment intent creator with the Stripe Node SDK
• validates amount, currency, and optional metadata before calling Stripe
• returns Stripe payment intent id/client secret and preserves Stripe error messages
• adds mocked unit tests plus an env-gated Stripe smoke test
• fixes the API test script glob so node:test discovers test files reliably

Demo

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

Tests

• npm test -w apps/api

Smoke test

The real Stripe API smoke test is guarded behind RUN_STRIPE_SMOKE_TESTS=1 and STRIPE_SECRET_KEY so it can run in test-mode environments without exposing keys.