Heroku Scheduler Transactional Email with Rovela (2026): One-Off Dynos + Config Vars + Idempotency-Key per Window
Wire Heroku Scheduler (every 10 minutes / hourly / daily one-off dynos) to Rovela POST /emails: config vars for sending_access, a one-shot command that exits, UTC daily slots, and a window-scoped Idempotency-Key so rare double-fires never duplicate digests.