fix: resolve critical/important bugs from code review (#1,#2,#4,#5,#6,#8)

- #1: OI division by zero — already fixed (prev_oi == 0.0 guard exists)
- #2: cumulative trade count used max() instead of sum(), breaking ML trigger
- #4: fetch_history API calls now retry 3x with exponential backoff
- #5: parquet upsert now deduplicates timestamps before sort
- #6: record_pnl() is now async with Lock for multi-symbol safety
- #8: exit_price=0.0 skips close handling with warning log

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/user_data_stream.py

@@ -94,6 +94,13 @@ class UserDataStream:
         net_pnl    = realized_pnl - commission
         exit_price = float(order.get("ap", "0"))
 
+        if exit_price == 0.0:
+            logger.warning(
+                f"[{self._symbol}] 청산 이벤트에서 exit_price=0.0 — "
+                f"ap 필드 누락 가능. 청산 처리 스킵 (rp={realized_pnl:+.4f})"
+            )
+            return
+
         if order_type == "TAKE_PROFIT_MARKET":
             close_reason = "TP"
         elif order_type == "STOP_MARKET":