Cron scheduler: evaluates crontab entries and enqueues jobs when due.

Cron state (last_execution) is persisted to known_crontabs so that
the scheduler survives process restarts without double-firing. Jobs are
enqueued with job_key_mode = unsafe_dedupe, so multiple scheduler instances
running in parallel will not produce duplicate queue entries.

The background polling thread evaluates all registered entries on each tick
and fires any whose next_run_at <= now. The poll interval is coarse
(default 10 s) because cron granularity is one minute.

Relies on the ReadableColumn protocol extensions defined in database.core
to return java.time.Instant values for TIMESTAMPTZ columns.

Cron scheduler that enqueues jobs via job-manager when schedules are due.

Each cron entry has an identifier (used as both task-identifier and job-key),
a standard UNIX 5-field cron expression, and optional per-entry job creation
overrides. Cron state (last_execution) is persisted to skivi.known_crontabs
so the scheduler survives restarts without double-firing.

Jobs are enqueued with job_key_mode = unsafe_dedupe: if the previous run's
job is still queued or in flight, the new enqueue is a no-op.

Typical usage:

  (def crontabs
    [{:identifier "daily-report"
      :schedule   "0 2 * * *"
      :spec       {:queue-name "maintenance" :max-attempts 3}}
     {:identifier "hourly-cleanup"
      :schedule   "0 * * * *"}])

  (def scheduler
    (-> (scheduler/create-scheduler sys crontabs emitter {:timezone "Europe/London"})
        scheduler/start!))

  ;; On application shutdown
  (scheduler/stop! scheduler 5000)

Standard events emitted:
  :cron/fired - a cron entry fired and a job was enqueued; :identifier in data

Malli schemas for the scheduler component.