Add check_pobsync_install so native deployments can run the same runtime
diagnostics from the terminal that are available in the Django Self Check view.
The command prints every check with status, returns a failing exit code when
install-critical checks fail, supports fail-on-warning for stricter automation,
and is documented in the installer output and README update flow.
Update the native installer so the pobsync service user gets journal
read access when the host exposes systemd-journal or adm groups.
Apply ownership and private directory modes to the configured backup
root, and reuse the existing environment backup root on reinstall so
production updates do not fall back to /backups.
Add a self-check for journal access and a host detail action that can
prepare missing backup directories for existing host configurations.
