#741 Move JanitorService from per-worker to system cron job

JanitorService is currently started in worker.py for highway_default queue workers. This means N workers = N JanitorService instances, each running partition maintenance every 24 hours. While idempotent, this is wasteful and follows the same anti-pattern as the cron job spawning bug. Fix: Create a system cron job for partition maintenance during bootstrap, remove JanitorService from worker.py.