feat: consolidate LG TV bridge — cached IP discovery, robust WoL, systemd deploy

Rework the MQTT bridge into a single deployable unit:
- Drop the self-SSH hop; run locally on steel141 (on the TV's LAN, has
  bscpylgtv, effectively always-on).
- Cache last-known TV IP and probe it directly; /24 ping-sweep only on a
  cache miss. Stops the 30s HA state poll from flapping/spamming the net.
- WoL now unicast (last-known IP) + broadcast, ports 9/7, 3 bursts.
- systemd unit (User=seth, EnvironmentFile for MQTT_PASSWORD), install.sh,
  env template, .gitignore. Removes the half-finished lgtv.sh.new.

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

lgtv-mqtt.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=LG TV MQTT bridge for Home Assistant
+Documentation=https://git.sethpc.xyz/Seth/lgtv
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=seth
+# MQTT_PASSWORD lives here, root-owned 0600, never committed. See lgtv.env.example.
+EnvironmentFile=/etc/lgtv/lgtv.env
+ExecStart=/usr/bin/python3 /usr/local/bin/lgtv_mqtt.py
+Restart=on-failure
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target