GHSA-pxcx-fv34-x9p5: nginx ignition has Unauthenticated Admin Account Creation via Onboarding Race Condition

## Summary `POST /api/users/onboarding/finish` is registered as **anonymous (unauthenticated)** and creates a user with **full ReadWrite admin permissions**. Because the handler uses a check-then-act (TOCTOU) pattern between the "onboarding already completed?" check and the user-creation write, with no atomic guard, a remote unauthenticated attacker who can reach an instance in its pre-onboarding state can create an administrator account for themselves — and concurrent requests can create multiple admin accounts in a single race. …