Require explicitly configured secure JWT secrets for server deployments and persist a random per-installation desktop secret across restarts. Preserve explicit environment overrides for strict server validation. Fixes#665.
Contributed by RobinZhiBin <robinzhi@gmail.com> (@RobinZhiBin). Preserve migration history validation by default and document controlled recovery. Reviewed with the related community fixes; 71 backend regression tests passed.
