优化号主交接超时卡死问题

- 号主交接超时(owner_timeout)后允许补提交交接说明，避免临时延误导致订单卡死
- 后台新增"重置交接"动作，可将超时订单恢复为待号主交接并刷新计时
- 交接超时改以进入待交接时刻为基准计算，新增 handoff_started_at 字段

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/internal/jobs/ordertimeout/job.go

@@ -275,7 +275,7 @@ func (j *Job) handleOwnerSubmitTimeout(ctx context.Context, now time.Time, cfg t
 	}
 	var rows []model.RentalOrder
 	err := j.db.WithContext(ctx).
-		Where("status = ? AND handoff_status = ? AND created_at <= ?", "pending_handoff", "pending_owner", now.Add(-time.Duration(cfg.OwnerSubmitTimeoutMinutes)*time.Minute)).
+		Where("status = ? AND handoff_status = ? AND COALESCE(handoff_started_at, created_at) <= ?", "pending_handoff", "pending_owner", now.Add(-time.Duration(cfg.OwnerSubmitTimeoutMinutes)*time.Minute)).
 		Order("id ASC").
 		Limit(100).
 		Find(&rows).Error