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/modules/order/handler_admin.go

@@ -53,6 +53,10 @@ func (h *Handler) AdminMarkAbnormal(c *gin.Context) {
 	h.adminAction(c, h.service.AdminMarkAbnormal, gin.H{"abnormal": true})
 }
 
+func (h *Handler) AdminResetHandoff(c *gin.Context) {
+	h.adminAction(c, h.service.AdminResetHandoff, gin.H{"reset": true})
+}
+
 func (h *Handler) AdminRefund(c *gin.Context) {
 	orderID, ok := parseID(c)
 	if !ok {