Implement Stripe PaymentIntent creation

[dannyward630]

/claim #1

Fixes #1

Summary

• replace the timestamp-based payment stub with Stripe paymentIntents.create() using STRIPE_SECRET_KEY
• validate amount, normalize/default currency, and validate optional metadata before calling Stripe
• return Stripe id as paymentId and client_secret as clientSecret
• preserve Stripe API error messages through a provider error and return validation/provider errors from the payment route
• add mocked service tests, route tests, and an opt-in Stripe test-mode smoke test

Verification

• npm test — 8 passed, 1 skipped guarded Stripe smoke test
• npm run build — root script prints package-specific build guidance
• npm run lint — root script reports no lint configured
• git diff --check

Smoke test

The live Stripe smoke test is intentionally skipped by default so CI does not need secrets. It can be run with:

RUN_STRIPE_SMOKE_TEST=1 STRIPE_SECRET_KEY=sk_test_... npm test -w apps/api

Demo note

This is a backend-only payment service change. I do not have a Stripe test key in this environment, so the PR demo is the mocked API/service test run plus the guarded live smoke-test path above rather than a live provider charge.