fix: 审批接口自动补全缺失的approval记录

旧ticket可能没有关联approval记录（创建时代码不同），
导致approve/reject报400 No pending approval。
现在admin/dba角色会自动创建approval记录再审批。

backend/internal/handler/ticket.go

@@ -255,11 +255,28 @@ func ApproveTicket(c *gin.Context) {
 	
 	now := time.Now()
 	
-	// Update approval record
+	// Update approval record — auto-create if missing (legacy tickets)
 	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 {
-		c.JSON(http.StatusBadRequest, gin.H{"detail": "No pending approval for you"})
-		return
+		// No approval record — check if user is authorized to review
+		_, userRole, _ := middleware.GetCurrentUser(c)
+		if userRole == "admin" || userRole == "dba" {
+			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)
+		} else {
+			c.JSON(http.StatusBadRequest, gin.H{"detail": "No pending approval for you"})
+			return
+		}
 	}
 	
 	approval.Action = &input.Action