(ops) Add terminal self-check command for native installs

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.

scripts/install-systemd

@@ -586,3 +586,4 @@ echo "Useful commands:"
 echo "  systemctl status pobsync-web pobsync-worker pobsync-scheduler"
 echo "  journalctl -u pobsync-worker -f"
 echo "  sudo -u $SERVICE_USER pobsync-manage check"
+echo "  sudo -u $SERVICE_USER pobsync-manage check_pobsync_install"