fix: timezone handling and prediction logic

- api/models.py: Fix timezone conversion in stats queries by calculating UTC ranges in Python instead of using SQL CONVERT_TZ. Ensure JSON serializable types.
- api/prediction_service.py:
    - Update ensure_predictions_for_month to check previous day (ET) to cover CST month start.
    - Update _generate_prediction_for_hour to skip saving unclosed candles (current hour).
- trader/auto_trade_polymarket.py: Update to use hourly_model_predictions (V2) table for storing live trading predictions.

trading_log.json

@@ -9,5 +9,14 @@
     "side": "DOWN",
     "price": 0.505,
     "probability": 0.5563886165618896
+  },
+  {
+    "timestamp": "2026-02-11T22:32:46.922211",
+    "success": false,
+    "error": "该市场已存在成功交易记录",
+    "prediction": "DOWN",
+    "probability": 0.0,
+    "market": "Bitcoin Up or Down - February 11, 9AM ET",
+    "market_slug": "bitcoin-up-or-down-february-11-9am-et"
   }
 ]