enhance(backend): チャートの処理を一つずつ行うことでDBの同時接続とタイムアウトを削減 (#15239)

* sync charts one-at-a-time to reduce database contention and timeouts

* fix merge resolve failure

* Update Changelog

* update changelog

* add comments

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Co-authored-by: Hazelnoot <acomputerdog@gmail.com>