Get Usage By Mode

Wave 4 (2026-05-06): 当月の instant vs reflective 内訳を返す.

課金ページの「料金内訳」セクションで使用. message_history.engine_metadata.mode を集計する. mode 未セットの行 (Wave 4 以前 / 移行漏れ) は instant 扱い.

Returns: { "instant": {"count": int, "amount_jpy": int, "unit_price": int}, "reflective": {"count": int, "amount_jpy": int, "unit_price": int}, "total_count": int, "total_amount_jpy": int, "period_start": iso str | null, }