When the database has no users, the UI redirects to /setup and prompts for creation of the first admin account. The setup endpoints are self-disabling — once any user exists, POST /setup/admin returns 403 and the frontend redirects /setup back to /login. The backend uses an atomic transaction to prevent race conditions on concurrent requests. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>