- Support comma-separated domains in auth.allowed_domain config
- Use full email as system username (user@domain.com -> user_domain_com)
to avoid collisions with reserved names and across domains
- Update both auth providers (google, email) for multi-domain display
- Add tests for username generation and update email auth tests
New pluggable auth provider that sends passwordless sign-in links.
Works with domain restriction (same as Google OAuth). Falls back to
showing the link in browser when SMTP is not configured (dev mode).