POST /nodes/request-update-all (admin) does what Update Node does for each
approved node: flags update_requested_at for the HTTPS updater and nudges it
over MQTT. Pending and rejected nodes are skipped. Admin-only button on the
Nodes page reports which nodes weren't reachable over MQTT.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
