fix: 行动方案库统计卡片—补全API+overdue计算

backend/check_date.py

@@ -0,0 +1,9 @@
+import datetime
+print("System date:", datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S'))
+print("UTC date:", datetime.datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S'))
+
+# What period_str would the dashboard/kpis endpoint use?
+from datetime import datetime as dt
+now = dt.now()
+period_str = now.strftime("%Y-%m")
+print("period_str for month query:", period_str)