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-display.service

@@ -3,6 +3,8 @@ Description=Infoscreen Display Manager
 Documentation=https://github.com/RobbStarkAustria/infoscreen_client_2025
 After=network.target graphical.target
 Wants=network-online.target
+# Publish an MQTT alert if systemd gives up restarting (StartLimitBurst exceeded).
+OnFailure=infoscreen-notify-failure@%n.service
 
 [Service]
 Type=simple