feat(api): add Stripe payment intents

[devnull37]

/claim #1

Summary

• replace the timestamp payment stub with a real Stripe PaymentIntent integration using STRIPE_SECRET_KEY
• validate amount, currency, and metadata before calling Stripe
• return Stripe payment id and client secret to callers
• preserve Stripe error messages while returning API validation failures cleanly
• add mocked service tests plus an explicit env-gated Stripe smoke test

Verification

pm run test -w apps/api

Demo

• The test output demonstrates mocked PaymentIntent creation, validation, Stripe error preservation, and the skipped smoke path unless STRIPE_PAYMENT_SMOKE_TEST=true and STRIPE_SECRET_KEY are provided.