feat: integrate Stripe SDK for payment intent creation

[niufox]

Summary

Implements the secure payment gateway by replacing the stub createPaymentIntent with real Stripe SDK integration. Closes #1

Changes Made

apps/api/src/services/paymentService.js

• Initialize Stripe client using STRIPE_SECRET_KEY from the existing env config
• Validate payload.amount as a positive integer (smallest currency unit, e.g. cents) — throws descriptive error if missing or invalid
• Default payload.currency to "usd" when not provided, consistent with existing behavior
• Call stripe.paymentIntents.create() with { amount, currency }
• Return clientSecret (mapped from paymentIntent.client_secret) and paymentId (mapped from paymentIntent.id)
• Remove stub pay_${Date.now()} id generation
• Catch and re-throw Stripe-specific errors (StripeCardError, StripeInvalidRequestError, etc.) with original error details

apps/api/src/controllers/paymentController.js

• Wrap createPaymentIntent call in try/catch
• Forward service errors to the client with appropriate HTTP status codes

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

• Add test coverage for input validation (null, undefined, negative, zero, float amounts)
• Add test coverage for currency default behavior

apps/api/package.json

• Add stripe dependency

Acceptance Criteria Checklist

• stripe npm package is installed and STRIPE_SECRET_KEY environment variable is used to initialise the client — no hardcoded keys
• payload.amount is required and must be a positive integer; function throws with a descriptive error if missing or invalid
• payload.currency defaults to "usd" if not provided
• A real stripe.paymentIntents.create() call is made with at minimum { amount, currency }
• The resolved value includes clientSecret and paymentId
• The stub pay_${Date.now()} id generation is removed
• Stripe errors are caught and re-thrown with the original Stripe error

[Adamchaua]

Monitoring this PR. For any approved payout: EVM 0x1ecab01075f3bdf1b56b7d849c8e28ef88943624; PayPal ckelvinkhanh32@gmail.com.

[niufox]

Friendly follow-up on this PR!

This implementation:

• Uses the official stripe Node.js SDK (v17.7.0)
• Full error handling with proper HTTP status codes
• Includes unit tests for validation and success cases
• All acceptance criteria from Issue Implement Stripe payment intent service #359 are met

Would love a review when you have a moment. Thanks!

[niufox]

💰 Payment info for bounty payout:

• PayPal: niufox@qq.com

Please use this PayPal account for any approved bounty reward. Thank you!

[niufox]

Closing: identified as fake bounty project, no actual payouts