feat(backend): runtime LAN-IP detection (netinfo helper) + manual override

The backend's bridge network can't see the host's NICs, so HOST_IP from
install.sh went stale silently after a DHCP change. Now:

- services/netinfo_helper.py runs as a new `netinfo` service (same backend
  image, network_mode: host): every 60s it picks the PHYSICAL LAN address
  (main-table default-route NIC if real hardware, else first real NIC with
  IPv4; never WireGuard/Tailscale/ZeroTier/bridges/veths, ignores 169.254)
  and writes it to the shared `netinfo` volume. Stdlib only. On Docker
  Desktop (linuxkit/WSL2 kernel) it reports "unsupported" instead of the
  VM's meaningless address.
- services/lan_ip.py: one resolver used by /api/system/status (lan_ip +
  lan_ip_info), the pairing QR and the cloud heartbeat's local_ip:
  override (pos_settings network.lan_ip_override) → live detection (ignored
  when older than 5 min) → HOST_IP. Flags `mismatch` when a pinned address
  is no longer on any of the machine's NICs.
- PUT /api/system/lan-ip-override (manager): set a private IPv4 or null to
  return to automatic; public/loopback/link-local/IPv6 rejected (422).
- cloud_sync._get_local_ip uses the resolver (no more socket trick that
  returned the container IP).

Tested: helper selection on a fake sysfs/route table (8 cases incl. VPN
default routes) + real ioctl/route parsing on a Linux kernel; resolver
priority/staleness/mismatch/validation (18 cases); isolated full stack:
HOST_IP fallback on Docker Desktop, override save/validate/auth, simulated
Linux detection incl. DHCP change and dead helper, heartbeat IP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -17,6 +17,19 @@ services:
       - ${DATA_PATH}:/app/data
       - ${LOGO_PATH}:/app/logo.png:ro
       - ${FISCAL_PATH}:/mnt/fiscal
+      - netinfo:/netinfo:ro
+
+  # Finds the server's LAN IP on the PHYSICAL network (never a VPN) every minute
+  # and shares it with the backend (pairing QR, manager, heartbeat). Needs the
+  # host's network namespace because the backend's bridge network can't see
+  # real NICs. Same image as the backend — see services/netinfo_helper.py.
+  netinfo:
+    image: ${REGISTRY}/pos-backend:${VERSION:-latest}
+    restart: unless-stopped
+    network_mode: host
+    command: ["python", "-m", "services.netinfo_helper"]
+    volumes:
+      - netinfo:/netinfo
 
   waiter_pwa:
     image: ${REGISTRY}/pos-waiter:${VERSION:-latest}
@@ -44,3 +57,6 @@ services:
       - waiter_pwa
       - manager_dashboard
     restart: unless-stopped
+
+volumes:
+  netinfo: