Check whether an unlock key is still valid (public)

Re-checks the stored key against the live entitlement on EVERY call, which is what makes a refund or dispute actually re-lock the app. Call it on each load and unlock only on { entitled: true }. IMPORTANT: only a 200 with entitled: false means unpaid , a 429 or 5xx means the check could not run, so keep the current state rather than re-locking a buyer over an outage.