feat: QR PNG 생성 + vote URL resolver

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -11,8 +11,11 @@ import os
 import random as _rand
 import threading
 from datetime import datetime
+import socket
+from io import BytesIO
 from pathlib import Path
 
+import qrcode
 import streamlit as st
 
 DATA_PATH = os.environ.get(
@@ -200,6 +203,35 @@ def fmt_team(team, titles):
     return f"{team} — {t}" if t else team
 
 
+def make_qr_png(url: str, box_size: int = 20) -> bytes:
+    img = qrcode.make(url, box_size=box_size, border=2)
+    buf = BytesIO()
+    img.save(buf, format="PNG")
+    return buf.getvalue()
+
+
+def _detect_lan_ip() -> str:
+    """LAN IP 자동 감지. 실패 시 'localhost'."""
+    try:
+        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        s.connect(("8.8.8.8", 80))
+        ip = s.getsockname()[0]
+        s.close()
+        return ip
+    except Exception:
+        return "localhost"
+
+
+def compute_vote_url() -> str:
+    data = load_data()
+    base = (
+        data.get("settings", {}).get("public_base_url")
+        or os.environ.get("PUBLIC_BASE_URL")
+        or f"http://{_detect_lan_ip()}:8501"
+    )
+    return f"{base.rstrip('/')}/?mode=vote"
+
+
 def compute_winners():
     """우선순위 기반 1팀 1상 + 동률 처리."""
     data = load_data()