feat(notifications): persistent read cursor + delivery history

notification_cursors table stores one read-cursor per user per tenant;
GET /v1/activity returns unreadCount + readAt, POST /v1/activity/read
persists the cursor. Reminder dispatches now append reminder.sent /
reminder.failed system events so delivery history reaches the feed.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/notifications/service.go

@@ -141,6 +141,19 @@ func (s *Service) DispatchDue(ctx context.Context, limit int) (domain.DispatchRe
 			return domain.DispatchReminderJobsResponse{}, err
 		}
 
+		action := "reminder.sent"
+		if status != "sent" {
+			action = "reminder.failed"
+		}
+		_ = s.repo.AppendActivityEvent(ctx, db.ActivityEventParams{
+			TenantID:   job.TenantID,
+			ActorKind:  "system",
+			Action:     action,
+			EntityType: "booking",
+			EntityID:   &job.BookingID,
+			Summary:    job.CustomerName + " — " + job.Channel,
+		})
+
 		if status == "sent" {
 			response.SentCount++
 		} else {