fix(notification): dedupe due/overdue events to prevent resend on scan

internal/notification/service.go

@@ -476,8 +476,7 @@ func (s *Service) PublishTaskEvent(ctx context.Context, eventType string, task T
 		traceID = eventID
 	}
 	if eventType == "task.overdue" {
-		bucket := time.Now().UTC().Unix() / int64(s.cfg.OverdueThrottle/time.Second)
-		eventID = fmt.Sprintf("task-overdue:%d:%d", task.ID, bucket)
+		eventID = fmt.Sprintf("task-overdue:%d:%s", task.ID, task.DueAt)
 		traceID = eventID
 	}
 	payload := map[string]any{
@@ -681,14 +680,40 @@ func (s *Service) scheduleDueEvents(ctx context.Context) {
 			continue
 		}
 		if dueAt.After(now) && dueAt.Before(now.Add(s.cfg.DueSoonWindow)) {
+			if s.hasExistingTaskEvent(ctx, task.ID, "task.due_soon", task.DueAt) {
+				continue
+			}
 			_ = s.PublishTaskEvent(ctx, "task.due_soon", task)
 		}
 		if dueAt.Before(now) {
+			if s.hasExistingTaskEvent(ctx, task.ID, "task.overdue", task.DueAt) {
+				continue
+			}
 			_ = s.PublishTaskEvent(ctx, "task.overdue", task)
 		}
 	}
 }
 
+func (s *Service) hasExistingTaskEvent(ctx context.Context, taskID int64, eventType, dueAt string) bool {
+	var exists bool
+	err := s.pool.QueryRow(ctx, `
+		SELECT EXISTS (
+			SELECT 1
+			FROM notification_events
+			WHERE event_type = $1
+				AND payload->>'task_id' = $2
+				AND payload->>'due_at' = $3
+		)`,
+		eventType,
+		strconv.FormatInt(taskID, 10),
+		dueAt,
+	).Scan(&exists)
+	if err != nil {
+		return false
+	}
+	return exists
+}
+
 func (s *Service) runDailySummaryScheduler(ctx context.Context) {
 	ticker := time.NewTicker(s.cfg.SchedulerTick)
 	defer ticker.Stop()