How Zerodha Kite API Auth Works:
1. Register app at kite.trade/developers
2. Set Redirect URL → http://localhost:3000/callback
3. Direct user to: https://kite.trade/connect/login?api_key=YOUR_KEY
4. After login, Zerodha redirects to: http://localhost:3000/callback?request_token=XXXX
5. Exchange request_token + sha256(api_key+request_token+api_secret) for access_token
6. Use access_token for all subsequent API calls