lan: repurpose onboard eth0 as an internal LAN port on br0

With wifi + Starlink + cellular as uplinks, the onboard GbE earns its keep
as a wired LAN port instead of the metric-150 ethernet WAN: eth0 leaves
netplan/NM (added to van-ap-unmanaged.conf) and joins br0 via networkd
(23-van-lan-eth0.network), so wired clients get 10.42.0.x DHCP/DNS and NAT
exactly like Wi-Fi clients. The eth WAN is gone from van-failover's config.

Cutover notes: delete the old netplan-eth0 / stray eth0 NM profiles, then
nmcli general reload + networkctl reload/reconfigure eth0. NEVER cable this
port (or the USB LAN dongle) back into an upstream LAN — dnsmasq on br0
would serve rogue DHCP there; STP stays off, so don't cable both wired LAN
ports to the same switch either.

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

failover/config.json

@@ -10,7 +10,6 @@
   ],
   "wans": [
     { "name": "wifi",     "device": "wlan0",           "metric": 100 },
-    { "name": "eth",      "device": "eth0",            "metric": 150 },
     { "name": "starlink", "device": "enxd8ec5eeb3512", "metric": 200 },
     { "name": "cellular", "connection": "Koodo",       "metric": 300 }
   ]