Issue a single-use signup nonce
Issues a short-lived, single-use nonce for the signup flow. The client
embeds the returned nonce in the purpose-bound signup message it signs,
then submits that proof to /v1/onboarding/signup, which consumes the
nonce exactly once. Nonces expire after expires_in seconds; expired,
already-consumed, or client-generated nonces are rejected at signup
with 401 NONCE_INVALID.
The request body is empty.
POST
Issue a single-use signup nonce
Issue a single-use signup nonce