Package apikey is the API-key token format, shared by the engine (mint and
resolve) and verify (routing a bearer token to the API-key path).
A token is <marker><lookup id>_<secret>. The marker is "<prefix>_st_", or
"st_" without a host prefix. The lookup id is public and indexed; only a
SHA-256 of the secret is stored. Both parts are base62, so the first "_"
after the marker is the delimiter.