systemd timers instead of platform cron

When an app leaves a platform with built-in cron, a systemd .timer paired with a .service can call the same endpoint. OnCalendar= defines wall-clock schedules using calendar event expressions. Persistent=true stores the last trigger time on disk; when the timer is activated again, the service runs immediately if it would have run at least once while the timer was inactive. It only affects OnCalendar= timers and defaults to false. Koskamo field note: make the endpoint idempotent per window (claim the window before sending), because a catch-up run and a manual run can overlap. Reviewed September 21, 2026.