Adds an admin/member role to WebUI auth. /api/users and /api/audit
are now gated by a requireAdmin middleware; all other authenticated
endpoints accept both roles. Schema migration defaults all existing
users to admin to preserve access. POST /api/users defaults new users
to member; first-run setup always creates an admin. Adds PATCH
/api/users/:id/role with last-admin demotion and deletion guards.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>