perf(api): derive past usage dates in one pass

[seonghobae]

Summary

• extracts the usagePerTurn past-date invalidation logic into a documented helper;
• derives de-duplicated UTC day-start ISO timestamps in one pass instead of chained map → filter → map allocations;
• adds focused regression coverage for empty input, duplicate timestamps, Korea-time/UTC boundary behavior, today/future exclusion, and invalid timestamp handling.

Why this replaces the unique part of #399

PR #399 was stale, non-mergeable, and mixed a bot journal with chart and API optimizations. The chart-side date parsing is now owned by PR #425, which carries a current-base test and the formatRelativeTimeFromMs contract. This PR preserves only the API-route delta on exact current developmental without the .jules artifact or chart churn.

Issues are disabled in this repository, so the remaining API work could not be preserved as a tracking issue before closing #399.

Behavior contract

The helper preserves existing endpoint semantics:

• duplicate timestamps on the same UTC day collapse to one ISO day-start value;
• Korea-time timestamps are normalized by UTC date, matching the old getUTC* logic;
• today and future UTC days are excluded from cache invalidation;
• invalid timestamps are ignored because they produce NaN and fail the ms < todayMs test, as in the previous callback path.

Verification

The PR is test-first in effect for the extracted contract: new focused derivePastUsageDateIsoStrings tests describe the behavior before the production route uses the helper. Hosted CI, central OpenCode coverage evidence, SAST/Semgrep, Security Scan, dependency review, and current-head review must run on this exact head before merge.

Merge gate

Merge only after exact-head CI, central OpenCode coverage evidence, SAST/Semgrep, Security Scan, dependency review, current-head review, zero unresolved actionable threads, and all live branch-protection rules pass. No check or review evidence from #399 transfers.

Summary by CodeRabbit

• 버그 수정

  • 과거 사용량 날짜를 UTC 기준으로 정확히 계산하도록 개선했습니다.
  • 오늘 및 미래 날짜, 잘못된 타임스탬프가 과거 사용량에 포함되지 않습니다.
  • 중복 날짜를 제거해 캐시 무효화 처리가 안정적으로 동작합니다.

• 테스트

  • 빈 입력, 날짜 중복, 한국 시간대 타임스탬프 등 주요 사례에 대한 검증을 추가했습니다.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 6c709a82-419b-49a0-b586-c461a3fe2073

📥 Commits

Reviewing files that changed from the base of the PR and between 1c1a161 and f2c919a.

📒 Files selected for processing (2)

• packages/web/src/app/api/events/route.test.ts
• packages/web/src/app/api/events/route.ts

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📝 Walkthrough

Walkthrough

사용량 타임스탬프에서 과거 UTC 날짜를 도출하는 로직을 내보내기 함수로 추출했다. POST 처리에서 해당 함수를 사용한다. 빈 입력, 중복 제거, 시간대 변환, 날짜 필터링, 잘못된 타임스탬프 처리를 테스트한다.

Changes

과거 사용량 날짜 처리

Layer / File(s)	Summary
UTC 과거 날짜 헬퍼와 POST 연결 packages/web/src/app/api/events/route.ts	사용량 타임스탬프에서 오늘 이전의 고유한 UTC 날짜 시작 ISO 문자열을 생성하는 derivePastUsageDateIsoStrings를 추가했다. POST는 인라인 계산 대신 이 헬퍼를 호출한다.
날짜 도출 동작 테스트 packages/web/src/app/api/events/route.test.ts	빈 입력, UTC 날짜별 중복 제거, 한국 시간대 타임스탬프 변환, 오늘 및 미래 날짜 제외, 잘못된 타임스탬프 무시를 검증한다.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to f2c91

This change consolidates past usage-date derivation while preserving the documented API behavior and adding focused regression coverage; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: claude, greatsumini

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	제목은 과거 사용량 날짜를 한 번에 도출하도록 변경한 핵심 내용을 명확하고 간결하게 설명합니다.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/events-past-date-one-pass

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[seonghobae]

Closing this PR as a duplicate of the stronger current-base canonical lane #446. After #447 was opened, the live PR sweep showed #446 already preserves the same server-side portion of #399 with a cleaner package boundary: collectPastUtcDates lives under packages/web/src/lib/server/usage-date-buckets.ts, returns Date[] directly for Prisma invalidation, handles invalid timestamps explicitly, and includes leap-day / ordering / timezone regression tests.

Keeping both would split one API optimization across competing branches. #446 should be the owner; it needs its PR body updated from the initial RED-stage wording, exact-head CI/security/coverage/review, and ready-for-review only after the current implementation is verified. No checks or review evidence from #447 transfers.