Returns a single-use session key for the 1-Click Signup embed, plus the
Verified Inc environment the embed must run against. Call this from your
backend and pass the session key to the embed: the key is the embed's
credential for this flow, while the Verified API key behind it never
leaves ProphetX.

Unlike every other embed, 1-Click Signup is not bootstrapped with a
token from GET /private/v1/tokens — it runs before the user it will
create exists, so there is no user to mint an embed token for. This
endpoint takes its place, and is the one call you make with a JWT that
omits sub (and therefore subsig and client_ip).

A session key is consumed by one embed instance — a reloaded embed
needs a fresh call.