fix: 审批流程三个bug修复

1. 点通过不再重复创建审批记录(后端无NULL记录时返回400而非自动创建)
2. 非提交人不再显示撤销按钮(列表页增加applicant_id校验)
3. 已审批工单不再显示通过/拒绝按钮(is_approver只计算pending记录)
4. 前端审批失败时弹出错误提示而非静默忽略

backend/internal/handler/ticket.go

@@ -138,7 +138,8 @@ func ListTickets(c *gin.Context) {
 		model.DB.Model(&model.TicketApproval{}).Where("ticket_id = ? AND step_type = 'execute'", t.ID).Count(&execCount)
 		var isExec, isAppr int64
 		model.DB.Model(&model.TicketApproval{}).Where("ticket_id = ? AND step_type = 'execute' AND approver_id = ?", t.ID, userID).Count(&isExec)
-		model.DB.Model(&model.TicketApproval{}).Where("ticket_id = ? AND step_type != 'execute' AND step_type != 'submit' AND approver_id = ?", t.ID, userID).Count(&isAppr)
+		// Only count PENDING approvals (action IS NULL) — already-approved records should not show approve button
+		model.DB.Model(&model.TicketApproval{}).Where("ticket_id = ? AND step_type != 'execute' AND step_type != 'submit' AND approver_id = ? AND action IS NULL", t.ID, userID).Count(&isAppr)
 		result = append(result, TicketWithMeta{
 			Ticket:         t,
 			HasAuditStep:   auditCount > 0,
@@ -496,24 +497,14 @@ func ApproveTicket(c *gin.Context) {
 
 	now := time.Now()
 
-	// Update approval record — auto-create if missing (legacy tickets)
+	// Find PENDING approval for this user (action IS NULL)
 	var approval model.TicketApproval
-	if err := model.DB.Where("ticket_id = ? AND approver_id = ? AND action IS NULL", id, userID).First(&approval).Error; err != nil {
-		// No approval record — authorized role can create one
-		stepName := "审核"
-		stepType := "audit"
-		approverRole := userRole
-		approval = model.TicketApproval{
-			TicketID:     ticket.ID,
-			StepNumber:   1,
-			StepName:     &stepName,
-			StepType:     &stepType,
-			ApproverID:   &userID,
-			ApproverRole: &approverRole,
-		}
-		model.DB.Create(&approval)
+	if err := model.DB.Where("ticket_id = ? AND approver_id = ? AND action IS NULL AND step_type != 'execute'", id, userID).First(&approval).Error; err != nil {
+		// Already approved or no pending approval for this user
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "没有待处理的审批记录（你可能已经审批过了）"})
+		return
 	}
-	
+
 	approval.Action = &input.Action
 	approval.Comment = input.Comment
 	approval.ActualApproverID = &userID