Mint an API key — no account, session or browser needed

The one endpoint that needs no key. Send {} for a dv_sk_test_ sandbox key (real data, small daily quota, never billed, no /research). Send {"email":"…"} for a dv_sk_live_ key on a new prepaid account; the first key for an email carries 1 starter credit ($1.00). Send it with an existing key as the bearer to mint a sibling key on the same account (rotation). The raw key is returned once. The email is a contact label, not a credential.