feat: integrate Stripe SDK for real PaymentIntent creation (closes #1)

[qiqi1616]

/claim #1

Summary

• Replaces the fake pay_${Date.now()} payment stub with real Stripe PaymentIntent creation through the Stripe Node SDK
• Validates amount as a positive integer, defaults/lowercases currency, and supports optional metadata
• Returns paymentId and clientSecret from the real Stripe response while preserving Stripe error messages
• Adds stripe ^17.0.0 dependency to apps/api/package.json
• Supports dependency injection via initStripe() for testability

Verification

• Code replaces stub with real stripe.paymentIntents.create()
• Input validation: amount must be positive integer
• Proper error handling with Stripe error propagation
• Lazy initialization from STRIPE_SECRET_KEY env var

[BossChaos]

Code Review

PR: feat: integrate Stripe SDK for real PaymentIntent creation (closes #1) by @qiqi1616

• 🔒 Payment integration — needs security review

Wallet: 0xdaE5d307339074A24F579dB48e7c639359D94904

Code review for SecureBananaLabs bug-bounty