fix: add retry with exponential backoff to OI/funding API calls (#4)

_fetch_oi_hist and _fetch_funding_rate had no retry logic, causing
crashes on rate limit (429) or transient errors during weekly report
data collection. Added _get_json_with_retry helper (max 3 attempts,
exponential backoff). Updated code-review docs to reflect completion.

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**: 부분 완료 (#1 기수정, #2/#4/#5/#6/#8 완료, #9 보류, #3/#7/#10~13 다음 스프린트)
+**상태**: 부분 완료 (#1/#2/#4/#5/#6/#8 완료, #9 보류, #3/#7/#10~13 다음 스프린트)
 
 ## 목표