Wallet authentication

Authentication is a CIP-30 signData challenge/response. The issuer hands the wallet a one-time challenge (POST /api/auth/challenge); the wallet returns a COSE_Sign1 envelope; the issuer verifies the signature against the declared stake address.

• No passwords or email — identity is control of a stake key.
• The public subject (sub) is a salted HMAC pseudonym, not a raw address.
• Admin sign-in uses the same primitive with owner keys, plus step-up re-signing for sensitive actions.

Zero custody — signatures are verified, never stored keys. The only private key the service manages is its own issuer signing key, encrypted at rest.