Implement Stripe payment gateway

[Cameron-xuan]

Closes #1

/claim #1

Demo

demos/stripe-payment-intent-demo.mov — 15s screen recording showing all 9 tests passing via npm test.

Changes

apps/api/src/services/paymentService.js

• Replaced stub createPaymentIntent with real Stripe SDK integration
• Input validation: amount required and must be a positive integer (cents); currency defaults to "usd"
• Calls stripe.paymentIntents.create({ amount, currency }) and returns { clientSecret, paymentId }
• Catches Stripe errors and re-throws with original message preserved
• Uses lazy singleton stripeClient (initialised from STRIPE_SECRET_KEY env var)

apps/api/src/controllers/paymentController.js

• Added try/catch: validation errors (status 400) return fail(res, message, 400); Stripe/unexpected errors propagate to errorHandler

apps/api/src/tests/payment.test.js

• 8 unit tests using injected mock Stripe client (no network calls)
• Verifies correct arguments passed to stripe.paymentIntents.create
• Covers: happy path, currency default, missing amount, zero/negative/float amount, Stripe error propagation
• 1 environment-gated integration smoke test (runs only when STRIPE_SECRET_KEY=sk_test_*)

apps/api/package.json

• Fixed test script: node --test src/tests/*.test.js (glob so all test files are discovered)

Test results

✔ GET /health returns ok payload
✔ returns clientSecret and paymentId from Stripe
✔ passes correct amount and currency to stripe.paymentIntents.create
✔ defaults currency to usd when omitted
✔ throws 400 error when amount is missing
✔ throws 400 error when amount is zero
✔ throws 400 error when amount is negative
✔ throws 400 error when amount is a float
✔ propagates Stripe API errors with original message
﹣ integration: creates real PaymentIntent in Stripe test mode # SKIP
pass 9, fail 0, skipped 1