feat: add notifier scheduler and NotifyOnce sent-tracking

internal/notifier/cron_schedule.go

@@ -0,0 +1,31 @@
+package notifier
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/robfig/cron/v3"
+)
+
+// CronSchedule wraps a robfig/cron schedule to satisfy the notifier.Schedule
+// interface used by StartScheduler. The spec follows the standard 5-field cron
+// syntax (e.g. "0 9 * * *" for daily at 09:00 in the process local time).
+type CronSchedule struct {
+	spec string
+	c    cron.Schedule
+}
+
+// NewCronSchedule parses a cron spec and returns a Schedule. An error is
+// returned if the spec is not a valid cron expression.
+func NewCronSchedule(spec string) (*CronSchedule, error) {
+	c, err := cron.ParseStandard(spec)
+	if err != nil {
+		return nil, fmt.Errorf("parse cron schedule %q: %w", spec, err)
+	}
+	return &CronSchedule{spec: spec, c: c}, nil
+}
+
+// Next returns the next time the schedule fires after t.
+func (s *CronSchedule) Next(t time.Time) time.Time {
+	return s.c.Next(t)
+}