feat(backend): identity advertises http_port (HTTP_PORT, default 80)

The native app downloads per-venue UI bundles from the plain-HTTP LAN port
(integrity via the sha256 in the manifest it fetched over pinned TLS), so it
needs to know the published port. compose passes HTTP_PORT to the backend.

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
@@ -53,6 +54,9 @@ def identity(db: Session = Depends(get_db)):
         "api_version": API_VERSION,
         # Encrypted LAN endpoint for the native app: https://<ip>:<port>, pin spki_sha256
         "tls": tls_info(),
+        # Plain-HTTP LAN port: the app downloads UI bundles from it (integrity via
+        # the sha256 in the manifest it fetched over TLS)
+        "http_port": int(os.environ.get("HTTP_PORT", "80")),
     }