feat(stock): GET /api/stock/holidays endpoint (SP-10 task-watcher용)

holidays.json(list) 노출. task-watcher가 휴장일 판정에 조회.
인증 불필요. 주말은 task-watcher가 weekday로 별도 판정.
Plan-B-Infra Phase 1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stock/app/main.py

@@ -162,6 +162,17 @@ def get_indices():
     """주요 지표(KOSPI 등) 실시간 크롤링 조회"""
     return fetch_major_indices()
 
+@app.get("/api/stock/holidays")
+def get_holidays():
+    """task-watcher가 조회하는 휴장일 목록. holidays.json(list) 노출 (인증 불필요)."""
+    try:
+        with open(_HOLIDAYS_PATH, encoding="utf-8") as f:
+            data = json.load(f)
+        holidays = data if isinstance(data, list) else []
+    except (OSError, ValueError):
+        holidays = []
+    return {"holidays": holidays}
+
 @app.post("/api/stock/scrap")
 def trigger_scrap():
     """수동 스크랩 트리거"""