Authenticated endpoint. Consumes the challenge issued by /auth/passkey/enroll, verifies the attestation via @simplewebauthn/server, and inserts a row into user_passkeys. A user-supplied friendly_name is stored alongside for UI display.
HttpOnly, Secure, SameSite=Lax cookie issued by /v1/auth/sign-in (or the SSO / email-callback flows). The value is an AES-256-GCM-encrypted blob of {access_token, refresh_token, issued_at, auth_method}. Browser clients include this automatically with credentials: "include".