Implement Stripe PaymentIntent creation

[alexgduarte]

/claim #1

Summary

• Replace the timestamp payment stub with Stripe PaymentIntent creation from STRIPE_SECRET_KEY.
• Validate amount, currency, and metadata before calling Stripe.
• Return Stripe paymentId/clientSecret and preserve Stripe provider error messages.
• Add mocked node:test coverage plus an opt-in live Stripe smoke test.

Verification

• npm.cmd test -w apps/api: 6 passed, 1 skipped live Stripe smoke test because no Stripe test key is present.
• npm.cmd test: 6 passed, 1 skipped live Stripe smoke test.
• git diff --check: no whitespace errors; CRLF warnings only from Git on Windows.

Smoke test note

The live Stripe smoke path is intentionally guarded by STRIPE_PAYMENT_SMOKE=true and STRIPE_SECRET_KEY so CI/local runs do not require payment credentials.