feat: remote commands, systemd units, process observability, broker auth split

- Command intake (reboot/shutdown) on infoscreen/{uuid}/commands with ack lifecycle
- MQTT_USER/MQTT_PASSWORD_BROKER split from identity vars; configure_mqtt_security() updated
- infoscreen-simclient.service: Type=notify, WatchdogSec=60, Restart=on-failure
- infoscreen-notify-failure@.service + script: retained MQTT alert when systemd gives up (Gap 3)
- _sd_notify() watchdog keepalive in simclient main loop (Gap 1)
- broker_connection block in health payload: reconnect_count, last_disconnect_at (Gap 2)
- COMMAND_MOCK_REBOOT_IMMEDIATE_COMPLETE canary flag with safety guard
- SERVER_TEAM_ACTIONS.md: server-side integration action items
- Docs: README, CHANGELOG, src/README, copilot-instructions updated
- 43 tests passing

scripts/infoscreen-notify-failure@.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=Infoscreen service-failed MQTT notifier (%i)
+# One-shot: run once and exit. %i is the failing unit name passed by OnFailure=.
+After=network.target
+
+[Service]
+Type=oneshot
+User=olafn
+Group=olafn
+WorkingDirectory=/home/olafn/infoscreen-dev
+EnvironmentFile=/home/olafn/infoscreen-dev/.env
+ExecStart=/home/olafn/infoscreen-dev/scripts/infoscreen-notify-failure.sh %i
+
+# Do not restart the notifier itself.
+Restart=no
+
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=infoscreen-notify-failure