实现结账流程：替换旧归还逻辑为完整结账-修正-争议流程

- 新增 order_checkouts 表，支持结账明细（消耗/押金扣除/退回/号主入账）
- 租客发起结账 → 号主确认 → 已完成（正常路径）
- 号主修改结账 → 租客确认修正 → 已完成（修正路径）
- 结账阶段任一方发起争议 → 后台仲裁 → 已完成/已关闭/异常（争议路径）
- 争议模块适配结账争议类型，仲裁结果新增 mark_abnormal
- 超时任务适配新的 pending_checkout_confirm 状态
- 前端结账明细面板、发起结账/确认/修正/拒绝表单全部实现
- 移除旧的 SubmitReturn/ConfirmReturn 接口

backend/internal/modules/order/handler.go

@@ -210,7 +210,7 @@ func (h *Handler) HandoffRecords(c *gin.Context) {
 	response.OK(c, gin.H{"items": items})
 }
 
-func (h *Handler) SubmitReturn(c *gin.Context) {
+func (h *Handler) SubmitCheckout(c *gin.Context) {
 	userID, ok := currentUserID(c)
 	if !ok {
 		response.Unauthorized(c, "缺少用户上下文")
@@ -220,12 +220,12 @@ func (h *Handler) SubmitReturn(c *gin.Context) {
 	if !ok {
 		return
 	}
-	var req SubmitReturnRequest
+	var req SubmitCheckoutRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
-		response.BadRequest(c, "归还说明不能为空")
+		response.BadRequest(c, "结账说明不能为空")
 		return
 	}
-	record, err := h.service.SubmitReturn(userID, id, req)
+	record, err := h.service.SubmitCheckout(userID, id, req)
 	if err != nil {
 		writeOrderError(c, err)
 		return
@@ -233,7 +233,7 @@ func (h *Handler) SubmitReturn(c *gin.Context) {
 	response.Created(c, record)
 }
 
-func (h *Handler) ConfirmReturn(c *gin.Context) {
+func (h *Handler) ConfirmCheckout(c *gin.Context) {
 	userID, ok := currentUserID(c)
 	if !ok {
 		response.Unauthorized(c, "缺少用户上下文")
@@ -243,7 +243,47 @@ func (h *Handler) ConfirmReturn(c *gin.Context) {
 	if !ok {
 		return
 	}
-	if err := h.service.ConfirmReturn(userID, id); err != nil {
+	if err := h.service.ConfirmCheckout(userID, id); err != nil {
+		writeOrderError(c, err)
+		return
+	}
+	response.OK(c, gin.H{"completed": true})
+}
+
+func (h *Handler) CounterCheckout(c *gin.Context) {
+	userID, ok := currentUserID(c)
+	if !ok {
+		response.Unauthorized(c, "缺少用户上下文")
+		return
+	}
+	id, ok := parseID(c)
+	if !ok {
+		return
+	}
+	var req CounterCheckoutRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "结账修正原因不能为空")
+		return
+	}
+	checkout, err := h.service.CounterCheckout(userID, id, req)
+	if err != nil {
+		writeOrderError(c, err)
+		return
+	}
+	response.OK(c, checkout)
+}
+
+func (h *Handler) AcceptCheckout(c *gin.Context) {
+	userID, ok := currentUserID(c)
+	if !ok {
+		response.Unauthorized(c, "缺少用户上下文")
+		return
+	}
+	id, ok := parseID(c)
+	if !ok {
+		return
+	}
+	if err := h.service.AcceptCheckout(userID, id); err != nil {
 		writeOrderError(c, err)
 		return
 	}
@@ -293,10 +333,16 @@ func writeOrderError(c *gin.Context, err error) {
 		response.Error(c, http.StatusConflict, "order_cannot_handoff", "当前订单不能交接")
 	case errors.Is(err, ErrOrderCannotReceive):
 		response.Error(c, http.StatusConflict, "order_cannot_receive", "当前订单不能确认收号")
-	case errors.Is(err, ErrOrderCannotReturn):
-		response.Error(c, http.StatusConflict, "order_cannot_return", "当前订单不能归还")
 	case errors.Is(err, ErrOrderCannotComplete):
 		response.Error(c, http.StatusConflict, "order_cannot_complete", "当前订单不能完成")
+	case errors.Is(err, ErrCheckoutCannotSubmit):
+		response.Error(c, http.StatusConflict, "checkout_cannot_submit", "当前订单不能发起结账")
+	case errors.Is(err, ErrCheckoutCannotConfirm):
+		response.Error(c, http.StatusConflict, "checkout_cannot_confirm", "当前结账不能确认")
+	case errors.Is(err, ErrCheckoutCannotCounter):
+		response.Error(c, http.StatusConflict, "checkout_cannot_counter", "当前结账不能修改")
+	case errors.Is(err, ErrInvalidCheckoutAmount):
+		response.BadRequest(c, "结账金额不符合规则")
 	case errors.Is(err, ErrPermissionDenied):
 		response.Error(c, http.StatusForbidden, "permission_denied", "无权操作该订单")
 	case IsNotFound(err):