paksenkin: TZ=Australia/Sydney made the Cache Maintenance job (and any repair
job) run every ~5s. Root cause: finished_at is written by SQLite CURRENT_TIMESTAMP
(always UTC) but the scheduler compared it against datetime.now() (naive LOCAL),
so the local↔UTC offset leaked into the elapsed time. Sydney (+11) made every job
look ~11h stale -> always due -> fired every poll; the Americas (behind UTC)
deflated it and masked the bug (why New_York 'worked').
Fix: compare in UTC. now = datetime.now(timezone.utc), and a new _hours_since()
helper parses the naive CURRENT_TIMESTAMP string AS UTC before subtracting — so
the machine timezone never affects scheduling. 5 tests incl. the literal repro
(a just-run job must not be due under Australia/Sydney) and a due-detection
sanity check; 41 repair-worker tests pass, ruff clean.
