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>

docs/plans/2026-03-07-code-review-improvements.md

@@ -1,7 +1,7 @@
 # 코드 리뷰 개선 사항
 
 **날짜**: 2026-03-07
-**상태**: 대기
+**상태**: 부분 완료 (#1 기수정, #2/#4/#5/#6/#8 완료, #9 보류, #3/#7/#10~13 다음 스프린트)
 
 ## 목표