Implement Stripe PaymentIntent creation for payments

[marklao666888]

Closes #1

Summary

• Replaced the placeholder payment intent implementation with the Stripe SDK.
• Added amount/currency validation, metadata normalization, and Stripe API error message preservation.
• Added Node test coverage with a mocked Stripe client plus an opt-in Stripe smoke test guarded by environment variables.

Validation

• node --check apps/api/src/services/paymentService.js
• node --check apps/api/src/tests/paymentService.test.js
• node --check apps/api/src/tests/paymentService.smoke.test.js
• node --test apps/api/src/tests/paymentService.test.js

[marklao666888]

Submitted the payment gateway implementation for #1.

This PR adds Stripe PaymentIntent creation, amount/currency validation, manual capture for escrow-style payments, platform fee calculation, Connect transfer support, and payment service tests.