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/risk_manager.py

@@ -52,9 +52,10 @@ class RiskManager:
             self.daily_pnl += pnl
             logger.info(f"포지션 종료: {symbol}, PnL={pnl:+.4f}, 누적={self.daily_pnl:+.4f}")
 
-    def record_pnl(self, pnl: float):
-        self.daily_pnl += pnl
-        logger.info(f"오늘 누적 PnL: {self.daily_pnl:.4f} USDT")
+    async def record_pnl(self, pnl: float):
+        async with self._lock:
+            self.daily_pnl += pnl
+            logger.info(f"오늘 누적 PnL: {self.daily_pnl:.4f} USDT")
 
     def reset_daily(self):
         """매일 자정 초기화"""