Backup shard (shard 3) is encrypted with user's WebAuthn passkey
public key (P-256 ECDH). HSM stores the ciphertext but cannot
decrypt — only user's biometric (via Secure Enclave private key) can.
ATS shard (2) + encrypted_backup (3) = company CANNOT sign.
User shard (1) + passkey_decrypt(3) = user CAN exit independently.
Added UserPasskeyFingerprint to WalletBackup model.
Backup handler looks up user's WebAuthn credential for encryption.