feat: systemd unit + env template + installer for MQTT bridge

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

install.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -euo pipefail
+HERE="$(cd "$(dirname "$0")" && pwd)"
+
+sudo install -m 0755 "$HERE/pos_briefing_mqtt.py" /usr/local/bin/pos_briefing_mqtt.py
+sudo install -d -m 0755 /etc/pos-briefing
+if [ ! -f /etc/pos-briefing/pos-briefing.env ]; then
+  sudo install -m 0600 "$HERE/pos-briefing.env.example" /etc/pos-briefing/pos-briefing.env
+  echo "!! Edit /etc/pos-briefing/pos-briefing.env and set MQTT_PASSWORD"
+fi
+sudo install -m 0644 "$HERE/pos-briefing-mqtt.service" /etc/systemd/system/pos-briefing-mqtt.service
+sudo systemctl daemon-reload
+sudo systemctl enable pos-briefing-mqtt.service
+echo "Installed. Start with: sudo systemctl start pos-briefing-mqtt.service"