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>

local_backend/routers/system.py

@@ -1,11 +1,11 @@
 import asyncio
 import ipaddress
 import json
-import os
 import socket
 import time
 from fastapi import APIRouter, Depends, HTTPException, Query
 from fastapi.responses import StreamingResponse
+from pydantic import BaseModel
 from sqlalchemy.orm import Session
 from typing import List
 
@@ -19,6 +19,7 @@ from models.product import Category, Product
 from models.table import Table, TableGroup
 from services import printer_service
 from services.cloud_sync import _sync_once, _push_menu_snapshot
+from services.lan_ip import OVERRIDE_KEY, resolve_lan_ip, validate_lan_ip
 from middleware.license_check import license_state
 from config import settings
 
@@ -37,12 +38,6 @@ def health():
 API_VERSION = 1
 
 
-def _lan_ip() -> str | None:
-    """The address phones should use. Only HOST_IP is trusted: inside Docker any
-    auto-detection returns the container's bridge IP, which phones can't reach."""
-    return os.environ.get("HOST_IP", "").strip() or None
-
-
 @router.get("/identity")
 def identity(db: Session = Depends(get_db)):
     """Public, unauthenticated. Lets a phone confirm which venue a server is
@@ -67,6 +62,8 @@ def system_status(db: Session = Depends(get_db), user: User = Depends(get_curren
         reachable = printer_service.check_printer(p.ip_address, p.port)
         printer_statuses.append({"id": p.id, "name": p.name, "reachable": reachable})
 
+    lan = resolve_lan_ip(db)
+
     licensed = license_state.get("licensed", True)
     locked = license_state.get("locked", False)
     lock_pending = license_state.get("lock_pending", False)
@@ -111,11 +108,37 @@ def system_status(db: Session = Depends(get_db), user: User = Depends(get_curren
         "last_sync": license_state.get("last_sync"),
         "waiter_domain": license_state.get("waiter_domain"),
         "site_id": settings.SITE_ID or None,
-        "lan_ip": _lan_ip(),
+        "lan_ip": lan.get("effective"),
+        "lan_ip_info": lan,
         "printers": printer_statuses,
     }
 
 
+class LanIpOverride(BaseModel):
+    ip: str | None = None  # null / empty → clear the override (back to automatic)
+
+
+@router.put("/lan-ip-override")
+def set_lan_ip_override(body: LanIpOverride, db: Session = Depends(get_db), user: User = Depends(require_manager)):
+    """Pin the server's LAN IP (what phones and the pairing QR use). Clearing it
+    returns to automatic detection / HOST_IP."""
+    row = db.query(PosSettings).filter(PosSettings.key == OVERRIDE_KEY).first()
+    if body.ip and body.ip.strip():
+        try:
+            value = validate_lan_ip(body.ip)
+        except ValueError as e:
+            raise HTTPException(status_code=422, detail=str(e))
+        if row:
+            row.value = value
+            row.updated_by_id = user.id or None
+        else:
+            db.add(PosSettings(key=OVERRIDE_KEY, value=value, updated_by_id=user.id or None))
+    elif row:
+        db.delete(row)
+    db.commit()
+    return resolve_lan_ip(db)
+
+
 @router.post("/sync-license")
 async def sync_license_now(user: User = Depends(require_manager)):
     """Trigger an immediate cloud heartbeat and return the fresh license state."""

local_backend/services/cloud_sync.py

@@ -16,7 +16,6 @@ import asyncio
 import json
 import logging
 import os
-import socket
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
 
@@ -80,18 +79,18 @@ def _compute_expiry_fields(expires_at_str: str | None) -> dict:
 
 
 def _get_local_ip() -> str | None:
-    """Best-effort detection of the host machine's LAN IP address.
-    When running inside Docker the socket trick returns the container/bridge IP,
-    so we honour HOST_IP if it is explicitly provided via the environment."""
-    import os
-    if host_ip := os.environ.get("HOST_IP", "").strip():
-        return host_ip
+    """The server's LAN IP as phones see it — same resolver as the manager's
+    pairing QR (override → live detection → HOST_IP), see services/lan_ip.py.
+    No socket tricks: inside Docker they return the unreachable bridge IP."""
+    from database import SessionLocal
+    from services.lan_ip import resolve_lan_ip
+    db = SessionLocal()
     try:
-        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
-            s.connect(("8.8.8.8", 80))
-            return s.getsockname()[0]
+        return resolve_lan_ip(db)["effective"]
     except Exception:
         return None
+    finally:
+        db.close()
 
 
 async def _sync_once():

local_backend/services/lan_ip.py

@@ -0,0 +1,91 @@
+"""
+The server's LAN address — the one phones use (http://<ip>), the pairing QR
+encodes and the cloud heartbeat reports. One resolver, used everywhere.
+
+Priority:
+  1. override  — set once in the manager (pos_settings 'network.lan_ip_override')
+  2. detected  — live, from the netinfo helper container (services/netinfo_helper.py)
+  3. env       — HOST_IP from .env (written by install.sh)
+  → None: the manager falls back to the address it was opened with.
+
+When the address in use (override / env) differs from what the helper sees
+right now, `mismatch` is set so the manager can warn before phones break
+(typical cause: the router's DHCP handed the server a new address).
+"""
+import ipaddress
+import json
+import os
+from datetime import datetime, timezone
+
+OVERRIDE_KEY = "network.lan_ip_override"
+NETINFO_FILE = os.environ.get("NETINFO_FILE", "/netinfo/host_ip.json")
+DETECTION_MAX_AGE_SECONDS = 300  # helper writes every 60s; older means it stopped
+
+
+def validate_lan_ip(value: str) -> str:
+    """Normalise and check an override: a private, non-loopback IPv4 address."""
+    try:
+        addr = ipaddress.ip_address(value.strip())
+    except ValueError:
+        raise ValueError("Μη έγκυρη διεύθυνση IP (π.χ. 192.168.1.50)")
+    if addr.version != 4 or not addr.is_private or addr.is_loopback or addr.is_link_local:
+        raise ValueError("Η διεύθυνση πρέπει να είναι IPv4 τοπικού δικτύου (π.χ. 192.168.x.x ή 10.x.x.x)")
+    return str(addr)
+
+
+def read_detected(path: str = NETINFO_FILE, now: datetime | None = None) -> dict | None:
+    """Latest helper result, or None if the helper isn't running / file is stale."""
+    try:
+        with open(path) as f:
+            data = json.load(f)
+        detected_at = datetime.fromisoformat(data["detected_at"])
+    except (OSError, ValueError, KeyError, TypeError):
+        return None
+    now = now or datetime.now(timezone.utc)
+    data["age_seconds"] = int((now - detected_at).total_seconds())
+    data["stale"] = data["age_seconds"] > DETECTION_MAX_AGE_SECONDS
+    return data
+
+
+def resolve_lan_ip(db=None, override: str | None = None, detected: dict | None = None,
+                   env: str | None = None) -> dict:
+    """Effective LAN IP + where it came from. Pass db to read the override setting;
+    the explicit arguments exist for tests."""
+    if db is not None and override is None:
+        from models.settings import PosSettings
+        row = db.query(PosSettings).filter(PosSettings.key == OVERRIDE_KEY).first()
+        override = row.value.strip() if row and row.value and row.value.strip() else None
+    if detected is None:
+        detected = read_detected()
+    if env is None:
+        env = os.environ.get("HOST_IP", "").strip() or None
+
+    live = detected if detected and not detected.get("stale") and detected.get("ip") else None
+    live_ip = live["ip"] if live else None
+    live_all = {c["ip"] for c in (live or {}).get("candidates", [])} | ({live_ip} if live_ip else set())
+
+    if override:
+        effective, source = override, "override"
+    elif live_ip:
+        effective, source = live_ip, "detected"
+    elif env:
+        effective, source = env, "env"
+    else:
+        effective, source = None, None
+
+    return {
+        "effective": effective,
+        "source": source,            # override | detected | env | None
+        "override": override,
+        "detected": live_ip,
+        "detected_candidates": (live or {}).get("candidates", []),
+        "detection": (
+            "unsupported" if detected and detected.get("unsupported")
+            else "stale" if detected and detected.get("stale")
+            else "ok" if live_ip
+            else "unavailable"
+        ),
+        "env": env,
+        # Using a fixed address that this machine no longer has on any NIC
+        "mismatch": bool(effective and source in ("override", "env") and live_ip and effective not in live_all),
+    }

local_backend/services/netinfo_helper.py

@@ -0,0 +1,134 @@
+"""
+Host LAN-IP detector — runs as its own container with `network_mode: host`.
+
+The backend lives in a Docker bridge network and can only see its container
+address, never the machine's real network cards. This helper shares the host's
+network namespace, finds the server's address on the PHYSICAL network
+(Ethernet/WiFi — never a WireGuard/Tailscale/ZeroTier tunnel, Docker bridge or
+veth) and writes it to a small JSON file on a volume the backend reads
+(services/lan_ip.py). It refreshes every minute, so a DHCP change shows up
+without anyone re-running install.sh.
+
+Selection (same rules as install.sh's detect_host_ip):
+  1. the interface of the main-table default route, if it is real hardware
+     (real NICs have /sys/class/net/<if>/device; tunnels and bridges don't);
+  2. otherwise the first real-hardware interface that has an IPv4 address.
+
+On Docker Desktop (Windows/Mac dev machines) "the host network" is Docker's own
+Linux VM, whose address means nothing to phones — the helper detects that and
+reports no IP instead of a wrong one.
+
+Standalone on purpose: standard library only, no app imports, no database.
+Run: python -m services.netinfo_helper
+"""
+import fcntl
+import json
+import os
+import platform
+import socket
+import struct
+import time
+from datetime import datetime, timezone
+
+OUT_FILE = os.environ.get("NETINFO_FILE", "/netinfo/host_ip.json")
+INTERVAL_SECONDS = int(os.environ.get("NETINFO_INTERVAL", "60"))
+SIOCGIFADDR = 0x8915
+
+
+def ipv4_of(ifname: str) -> str | None:
+    """Primary IPv4 address of an interface, or None if it has none."""
+    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
+        try:
+            packed = fcntl.ioctl(s.fileno(), SIOCGIFADDR, struct.pack("256s", ifname[:15].encode()))
+        except OSError:
+            return None
+    return socket.inet_ntoa(packed[20:24])
+
+
+def default_route_iface(route_file: str = "/proc/net/route") -> str | None:
+    """Interface of the main routing table's default route (lowest metric)."""
+    best = None
+    try:
+        with open(route_file) as f:
+            next(f)  # header
+            for line in f:
+                cols = line.split()
+                if len(cols) < 7:
+                    continue
+                iface, dest, flags, metric = cols[0], cols[1], int(cols[3], 16), int(cols[6])
+                if dest == "00000000" and flags & 0x1:  # default route, RTF_UP
+                    if best is None or metric < best[1]:
+                        best = (iface, metric)
+    except OSError:
+        return None
+    return best[0] if best else None
+
+
+def detect(sys_net: str = "/sys/class/net", route_file: str = "/proc/net/route", ipv4=ipv4_of) -> dict:
+    """Pick the physical LAN address. Pure apart from the injected lookups (testable)."""
+    try:
+        names = sorted(os.listdir(sys_net))
+    except OSError:
+        names = []
+    candidates = []
+    for name in names:
+        if not os.path.exists(os.path.join(sys_net, name, "device")):
+            continue  # tunnel, bridge, veth, loopback
+        addr = ipv4(name)
+        if addr and not addr.startswith("169.254."):
+            candidates.append({"iface": name, "ip": addr})
+
+    default_if = default_route_iface(route_file)
+    chosen = next((c for c in candidates if c["iface"] == default_if), None)
+    if chosen is None and candidates:
+        chosen = candidates[0]
+    return {
+        "ip": chosen["ip"] if chosen else None,
+        "iface": chosen["iface"] if chosen else None,
+        "candidates": candidates,
+        "default_iface": default_if,
+    }
+
+
+def docker_desktop() -> bool:
+    """Docker Desktop's VM kernel identifies itself; its 'host' network isn't the LAN."""
+    release = platform.release().lower()
+    return "linuxkit" in release or "microsoft" in release
+
+
+def write_atomic(path: str, data: dict) -> None:
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    tmp = f"{path}.tmp"
+    with open(tmp, "w") as f:
+        json.dump(data, f)
+    os.replace(tmp, path)
+
+
+def run_once() -> dict:
+    if docker_desktop():
+        result = {"ip": None, "iface": None, "candidates": [], "default_iface": None,
+                  "unsupported": "docker-desktop"}
+    else:
+        result = detect()
+    result["detected_at"] = datetime.now(timezone.utc).isoformat()
+    write_atomic(OUT_FILE, result)
+    return result
+
+
+def main() -> None:
+    last_ip = object()
+    while True:
+        try:
+            result = run_once()
+            if result["ip"] != last_ip:
+                print(f"netinfo: LAN IP {result['ip']} via {result['iface']} "
+                      f"(candidates: {result['candidates']}{', ' + result['unsupported'] if result.get('unsupported') else ''})",
+                      flush=True)
+                last_ip = result["ip"]
+        except Exception as e:  # never die — the backend treats a stale file as "unknown"
+            print(f"netinfo: detection failed: {e}", flush=True)
+        time.sleep(INTERVAL_SECONDS)
+
+
+if __name__ == "__main__":
+    main()