Background maintenance scheduler.

Runs two classes of work on separate cadences:
  Short interval (maintenance-interval-ms, default 60 s):
    reset-locked-jobs — frees jobs orphaned by crashed workers
    refill-rate-limits — refills token buckets whose window has expired

  Cron schedule (schedule, default '0 3 * * *'):
    GC tasks listed in :tasks config (gc-task-identifiers, gc-job-queues,
    gc-job-history).

The GC schedule is evaluated on each maintenance tick; the first tick
after a scheduled time triggers the run. Last-run time is tracked in memory
and resets to nil on restart (causing GC to wait for the next scheduled slot).

Maintenance scheduler: reset-locked-jobs and refill-rate-limits run on a short
interval; GC tasks run on a configurable cron schedule.

Malli schemas for the maintenance component.