gps: modem GNSS -> gpsd -> OwnTracks/MQTT publisher

EC25-AF GNSS wiring (udev hotplug into gpsd) plus van-gps-owntracks, a
port of the wayback-era gps_to_owntracks.py: apt-only deps (python3-gps
instead of pip-only gpsdclient, paho 2.x callback API), broker secrets
moved out of the code into /etc/van-gps/config.json (0600, seeded from
a sanitized example), gpsd host now localhost. client_id is vanq-wan —
a legacy client still holds vanq on the broker and shared IDs get
kicked in a connect/disconnect loop. Exit-on-disconnect + systemd
Restart=always is the reconnect logic.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gps/van-gps-owntracks.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Publish gpsd fixes to OwnTracks via MQTT
+After=network-online.target gpsd.service
+Wants=network-online.target
+
+[Service]
+ExecStart=/usr/local/sbin/van-gps-owntracks
+# The script exits on MQTT disconnect / DNS failure by design; a fresh start
+# is the reconnect. 15s so a dead WAN doesn't turn this into a hot loop.
+Restart=always
+RestartSec=15
+
+[Install]
+WantedBy=multi-user.target