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/gpsd.default

@@ -0,0 +1,9 @@
+# Devices gpsd should collect to at boot time.
+# Empty: the EC25 NMEA port is hot-added via udev (77-modem-gps.rules -> gpsdctl@).
+DEVICES=""
+
+# -n: read GNSS data even with no clients connected (keeps the fix warm).
+GPSD_OPTIONS="-n"
+
+# Automatically hot add/remove USB GPS devices via gpsdctl (our udev rule uses this).
+USBAUTO="true"