fix(stocks): Wikipedia 403 우회를 위해 User-Agent 헤더 추가

pd.read_html() 대신 urllib로 HTML 직접 fetch 후 파싱.
User-Agent 미설정 시 Wikipedia가 403 반환하는 문제 수정.

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

app/services/index_constituents_service.py

@@ -5,7 +5,9 @@ S&P 500 / Nasdaq 100 구성 종목을 Wikipedia에서 조회
 
 import asyncio
 import logging
+from io import StringIO
 from typing import Optional
+from urllib.request import Request, urlopen
 
 import pandas as pd
 
@@ -41,7 +43,10 @@ class IndexConstituentsService:
         industry_col = config["industry_col"]
 
         logger.info(f"Fetching Wikipedia tables from {url}")
-        tables = pd.read_html(url, flavor="lxml")
+        req = Request(url, headers={"User-Agent": "Mozilla/5.0 (compatible; StockOracle/1.0)"})
+        with urlopen(req, timeout=25) as resp:
+            html = resp.read().decode("utf-8")
+        tables = pd.read_html(StringIO(html), flavor="lxml")
 
         # Try configured table index first
         df = None