Extend user-agent authentication to support Ed25519, ECDSA (secp256k1), and RSA (PSS+SHA-256) with minimal protocol and storage changes. Add key_type to auth requests and useragent_client, update key parsing/signature verification paths, and keep backward compatibility by treating UNSPECIFIED as Ed25519.
Remove key_identity indirection table, storing public keys and nonces
directly on client tables. Replace AuthResponse with AuthOk, add a
BootstrapActor to manage token lifecycle, and move user agent stream
handling into the actor module.
