Lay stake and profit for a single selection using your back stake, back odds, lay odds, and exchange commission.
Balances a real-money back and lay so both outcomes are as close as possible (small qualifying loss with commission).
API: same-origin /api (proxy → backend; set API_PROXY_TARGET if needed) — NEXT_PUBLIC_API_BASE_URL for direct URL; API_PROXY_TARGET for the proxy target (Next only).