Add payment gateway integration (Stripe/PayPal)

Integrate payment processing for orders.

Payment Providers:

• Stripe (primary)
• PayPal (alternative)

Features:

• Payment intent creation
• Payment confirmation handling
• Webhook processing for payment events
• Refund processing
• Payment failure handling
• PCI compliance considerations

Security:

• Never store card details
• Use tokenization
• Implement 3D Secure
• Audit all payment transactions

Testing:

• Use Stripe test mode
• Add payment simulation tests
• Test refund flows