Add authentication to a new app. Use for "add auth", "add login", Neon Auth (Managed Better Auth), identity routing, sign-up, sign-in, password reset, email OTP, magic links, organizations, phone OTP, OAuth, passkeys, MFA, trusted domains, invalid domain, and @neondatabase/auth. No existing identity: default to Managed Better Auth. Keep working Better Auth, Clerk, Supabase Auth, or another IdP. User asked to migrate from Supabase Auth: Managed Better Auth. A required plugin outside Managed support: self-managed Better Auth on a Neon Function or the existing app host. Also use for auth APIs in @neondatabase/neon-js.