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/test_holidays_endpoint.py

@@ -0,0 +1,22 @@
+"""GET /api/stock/holidays — task-watcher 휴장일 조회용."""
+from fastapi.testclient import TestClient
+from app.main import app
+
+client = TestClient(app)
+
+
+def test_holidays_returns_list():
+    r = client.get("/api/stock/holidays")
+    assert r.status_code == 200
+    data = r.json()
+    assert "holidays" in data
+    assert isinstance(data["holidays"], list)
+
+
+def test_holidays_entries_are_iso_dates():
+    r = client.get("/api/stock/holidays")
+    holidays = r.json()["holidays"]
+    if holidays:
+        import datetime as dt
+        for h in holidays[:5]:
+            dt.date.fromisoformat(h)   # raise 안 하면 통과