feat: integrate Stripe PaymentIntent in payment service

[szlandywang-sketch]

Summary

Replaces the stub createPaymentIntent with a real Stripe SDK integration.

Changes

• Install stripe npm package in apps/api workspace
• Rewrite paymentService.js to use real stripe.paymentIntents.create()
• Add input validation: amount required, must be positive integer (cents)
• Default currency to "usd" when not provided
• Return clientSecret and paymentId from Stripe response
• Remove stub pay_${Date.now()} ID generation
• Handle Stripe API errors with original messages preserved
• Add comprehensive test suite:
  • Unit tests for validation (missing amount, negative/zero/float values)
  • Controller tests (HTTP 400 responses)
  • Environment-guarded integration test (Stripe test mode)

Test Results

✔ createPaymentIntent validation - missing amount
✔ createPaymentIntent validation - amount must be positive integer
✔ createPaymentIntent validation - amount accepts valid positive integer
✔ createPaymentIntent defaults currency to usd
✔ POST /api/payments - returns 400 on missing amount
✔ POST /api/payments - returns 400 on invalid amount
﹣ Stripe integration - creates PaymentIntent in test mode (SKIP - requires env flag)

6 pass, 1 skip, 0 fail

Acceptance Criteria

• stripe npm package installed, STRIPE_SECRET_KEY env var used (no hardcoded keys)
• payload.amount required, positive integer — throws ValidationError otherwise
• payload.currency defaults to "usd"
• Real stripe.paymentIntents.create() call with { amount, currency }
• Returns clientSecret and paymentId
• Stub pay_${Date.now()} removed
• Stripe errors caught, original message preserved
• Unit tests mock Stripe SDK (via ConfigurationError when unconfigured)
• Integration test guarded by RUN_STRIPE_INTEGRATION_TESTS env flag

Closes #1

/bounty $350