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/model/order.go

@@ -14,6 +14,7 @@ type RentalOrder struct {
 	OwnerID                   uint64         `gorm:"not null;index" json:"owner_id"`
 	RenterID                  uint64         `gorm:"not null;index" json:"renter_id"`
 	RentedAt                  *time.Time     `json:"rented_at"`
+	HandoffStartedAt          *time.Time     `json:"handoff_started_at"`
 	EstimatedDurationHours    int            `gorm:"not null;default:24" json:"estimated_duration_hours"`
 	RentAmountCent            int64          `gorm:"not null;default:0" json:"-"`
 	OwnerRentAmountCent       int64          `gorm:"not null;default:0" json:"-"`