Permanently delete the authenticated user's account
Permanently erases the dashboard user account belonging to the caller.
This is a self-service GDPR Article 17 right-to-erasure for the
signed-in user.
Requires a fresh password re-authentication in the request body. SSO-
only accounts (no local password) cannot use this endpoint and must
contact support — those callers receive `400`.
Rate-limited per account to prevent password-guessing.