Add offline workflow and coverage tests

tests/test_downloader.py

@@ -1,6 +1,16 @@
+import os
+
 import pytest
 from data.downloader import download_daily
 
+pytestmark = [
+    pytest.mark.network,
+    pytest.mark.skipif(
+        os.environ.get("CEQ_RUN_LIVE_DOWNLOADER") != "1",
+        reason="set CEQ_RUN_LIVE_DOWNLOADER=1 to run live baostock/akshare smoke tests",
+    ),
+]
+
 
 def test_download_single_stock():
     """Smoke test: download data for 浦发银行 for a short window."""