fix(stats): compute week/month cutoffs at call time (#13789)

[LalitNarayanYadav]

Closes #13789

fix: week/month stats cutoffs are now computed when used, not at import time.

Technical

The constants DATE_ONE_DAY_AGO, DATE_ONE_WEEK_AGO, DATE_ONE_MONTH_AGO and DATE_ONE_YEAR_AGO in utils/dateutil.py were evaluated once when the module loaded, so a long-running server kept using its start date.

I turned them into functions (date_one_week_ago() etc.) and updated every place that used them: the summary() methods in core/ and reading_log_leaderboard in views/loanstats.py. The month calculation is unchanged; I'm leaving the month-length question for a separate issue.

Testing

Added test_date_cutoffs_are_computed_at_call_time in openlibrary/utils/tests/test_dateutil.py. It moves a fake clock forward a month and checks the cutoffs follow it; it fails on the old code.

I ran pytest openlibrary/utils/tests/test_dateutil.py locally (6 passed). I did not run the full suite or load the stats page, so I'm relying on CI for those.

Screenshot

N/A (no UI change)

Stakeholders

[openlibrary-bot]

Thanks for the PR, @LalitNarayanYadav!

🤖 A reviewer has been assigned for an initial pass.

PR triage checklist (maintainers / Pierre)

• PR description — not empty; explains what the change does and how to verify it
• References an issue — PR body contains a #NNN reference
  • Linked issue is triaged — has a Priority: * label (not just Needs: Triage)
  • Linked issue is assigned — has at least one assignee
• Commit history clean — no WIP/fixup/conflict noise; commit messages are meaningful
• CI passing — no failing check-runs
• Test cases present — if the change touches substantive logic, test coverage exists or is explained
• Proof of testing — PR body includes a description of what was tested, a screenshot, or a video

---

Note

This comment was automatically generated by Pierre, Open Library's PR first-touch assistant, on behalf of @mekarpeles. This is NOT a code review or sign-off.