Implement Razorpay payment integration

[darshacharya]

Description

The donation system currently uses a mock payment gateway that always succeeds. Replace it with real Razorpay integration for production use.

Current State

• backend/app/services/payment.py defines a PaymentGateway protocol and MockPaymentGateway
• backend/app/routers/donations.py calls payment_gateway.create_payment() but never verifies
• Donations are immediately marked as COMPLETED without actual payment

Requirements

Backend

• Implement RazorpayGateway class in backend/app/services/payment.py
• Add POST /api/payments/create-order to create a Razorpay order
• Add POST /api/payments/verify to verify payment signature after completion
• Only mark donation as COMPLETED and update raised_amount after verification
• Add Razorpay credentials to config (RAZORPAY_KEY_ID, RAZORPAY_KEY_SECRET)
• Keep MockPaymentGateway as default for development

Frontend

• Integrate Razorpay checkout.js in the donate modal
• Handle payment success/failure callbacks
• Show payment status feedback

Tech Notes

• Razorpay docs: https://razorpay.com/docs/
• Package: razorpay (Python SDK)
• Frontend: Load Razorpay script dynamically

Acceptance Criteria

• Razorpay order is created server-side
• Frontend opens Razorpay checkout
• Payment signature is verified server-side
• Donation only completes after verified payment
• Mock gateway still works for local development
• Failed payments are handled gracefully