Add OperatorUser SQLAlchemy model (operator_users table, auto-created by
create_all) with email uniqueness, default active/must_change_password/
failed_login_count, and sessions_valid_from truncated to whole seconds.
Add backend/operator_auth.py with feature flag, cookie constants, _ROLE_RANK
map, role_at_least(), and _norm_email() helpers.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
