Implement Stripe payment intents

[s0584273828-ctrl]

/claim #1

Summary

• Replace the timestamp pay_${Date.now()} stub with Stripe PaymentIntent creation via the official stripe SDK and STRIPE_SECRET_KEY.
• Validate positive-integer amount, default and validate currency, normalize metadata, and map Stripe id / client_secret to paymentId / clientSecret.
• Preserve Stripe provider error messages through API error handling and add mocked service tests plus an opt-in live smoke test.

Validation

• npm test -w apps/api -> 5 passed, 1 guarded Stripe smoke skipped without STRIPE_SECRET_KEY
• node --check apps/api/src/services/paymentService.js
• node --check apps/api/src/tests/paymentService.test.js
• git diff --check

Note: the live Stripe smoke path is committed but was not executed here because no Stripe test secret is available in this environment.

Prepared with AI assistance and manually reviewed before submission.