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/config.json.example

@@ -0,0 +1,13 @@
+{
+  "broker": "home.wrede.ca",
+  "port": 1883,
+  "username": "CHANGE_ME",
+  "password": "CHANGE_ME",
+  "client_id": "vanq-wan",
+  "topic": "owntracks/rv/gps",
+  "tid": "rv",
+  "gpsd_host": "127.0.0.1",
+  "idle_delay_s": 600,
+  "move_trigger_m": 250,
+  "mqtt_version": "3"
+}