feat(backend): public /api/system/identity for pairing + site_id/lan_ip in status

GET /api/system/identity (unauthenticated, license-exempt) returns
{app: 'xenia-pos', site_id, venue_name, version, api_version: 1} so a phone
can confirm which venue a server is when pairing and when rediscovering the
server after an IP change. No secrets: SITE_KEY never leaves the server.
/api/system/status now also returns site_id and lan_ip (HOST_IP only - in
Docker any auto-detected address is the unreachable bridge IP).

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

local_backend/routers/system.py

@@ -1,6 +1,7 @@
 import asyncio
 import ipaddress
 import json
+import os
 import socket
 import time
 from fastapi import APIRouter, Depends, HTTPException, Query
@@ -13,6 +14,7 @@ from models.printer import Printer
 from schemas.printer import PrinterCreate, PrinterUpdate, PrinterOut
 from routers.deps import get_current_user, require_manager, require_sysadmin
 from models.user import User
+from models.settings import PosSettings
 from models.product import Category, Product
 from models.table import Table, TableGroup
 from services import printer_service
@@ -30,6 +32,32 @@ def health():
     return {"status": "ok", "version": settings.VERSION}
 
 
+# Bump when the waiter app <-> backend contract changes incompatibly, so native
+# apps (which ship their own UI) can tell they're talking to a server they don't support.
+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
+    (pairing, and rediscovery after the server's IP changes). No secrets here:
+    site_id is an identifier — the secret is SITE_KEY, which never leaves the server."""
+    venue = db.query(PosSettings).filter(PosSettings.key == "venue.name").first()
+    return {
+        "app": "xenia-pos",
+        "site_id": settings.SITE_ID or None,
+        "venue_name": (venue.value if venue and venue.value else None),
+        "version": settings.VERSION,
+        "api_version": API_VERSION,
+    }
+
+
 @router.get("/status")
 def system_status(db: Session = Depends(get_db), user: User = Depends(get_current_user)):
     from datetime import datetime, timezone
@@ -82,6 +110,8 @@ def system_status(db: Session = Depends(get_db), user: User = Depends(get_curren
         "sync_failed": license_state.get("sync_failed", False),
         "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(),
         "printers": printer_statuses,
     }