Create Subscription Checkout Session

Creates a hosted Dodo Payments checkout session for a subscription product. `productKey` and `interval` are validated against a fixed whitelist (the product catalog) before being handed off to Dodo — arbitrary product/interval combinations are rejected with `400`. A Redis lock prevents starting a second checkout while one is already in flight. Session-authenticated only; blocked for non-admins in production during the current rollout phase.

curl -X POST "https://example.com/api/v1/billing/checkout" \  -H "Content-Type: application/json" \  -d '{    "productKey": "portfolio_pro",    "interval": "monthly"  }'

{  "success": true,  "message": "string",  "data": {    "url": "string"  }}