[Rahul-cs50]

Closes #4182

Rationale for this change

expire_snapshots().older_than(dt) converts the cutoff with datetime_to_millis, which reads a naive datetime as UTC. The docs call it as older_than(datetime.now() - timedelta(days=3)), and datetime.now() is a naive local time, so east of UTC the cutoff lands later than asked (in Tokyo, datetime.now() - timedelta(hours=1) expires snapshots that are seconds old) and west of UTC it lands earlier.

older_than() now reads a naive datetime as local time, as datetime.timestamp() does, by calling astimezone() on it before the conversion. An aware datetime is handled as before. datetime_to_millis itself is unchanged, since a naive value there means UTC for timestamp columns.

Are these changes tested?

Yes. test_expire_snapshots_older_than_reads_naive_datetime_as_local_time in tests/table/test_expire_snapshots.py sets TZ to Asia/Tokyo and checks that a naive cutoff one minute before a snapshot keeps it and one minute after expires it. It fails on main and passes with this change (it is skipped where time.tzset is unavailable). The issue's script now keeps all three snapshots under Asia/Tokyo, America/Los_Angeles and UTC. tests/table and tests/utils: 768 passed. ruff check and ruff format --check are clean.

Are there any user-facing changes?

older_than() with a naive datetime now uses the local time zone instead of UTC. Callers who passed naive UTC datetimes on purpose should pass an aware one (datetime.now(timezone.utc)).

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