fix: 处理域名重复申请问题，支持重试失败/过期的证书

backend/handlers/cert.go

@@ -78,7 +78,40 @@ func (h *CertHandler) CreateCertificate(c *gin.Context) {
 	// Check if domain already exists
 	var existing config.Certificate
 	if err := config.DB.Where("domain = ?", req.Domain).First(&existing).Error; err == nil {
-		c.JSON(http.StatusConflict, gin.H{"error": "domain already exists"})
+		// Domain exists, check if it's a failed/expired certificate that can be retried
+		if existing.Status == "error" || existing.Status == "expired" || existing.Status == "pending" {
+			// Update existing record and retry issuance
+			existing.Email = req.Email
+			existing.Provider = req.Provider
+			existing.ChallengeType = req.ChallengeType
+			existing.DNSProvider = req.DNSProvider
+			existing.DNSConfig = req.DNSConfig
+			existing.Status = "pending"
+			existing.ErrorMessage = ""
+			if req.AutoRenew != nil {
+				existing.AutoRenew = *req.AutoRenew
+			}
+			if req.RenewDays != nil {
+				existing.RenewDays = *req.RenewDays
+			}
+			if err := config.DB.Save(&existing).Error; err != nil {
+				c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+				return
+			}
+			// Start issuance in background
+			go func() {
+				if err := services.GetACMECertificate(&existing, h.Cfg); err != nil {
+					existing.Status = "error"
+					existing.ErrorMessage = err.Error()
+				} else {
+					existing.Status = "active"
+				}
+				config.DB.Save(&existing)
+			}()
+			c.JSON(http.StatusAccepted, gin.H{"message": "certificate re-issuance started", "certificate": existing})
+			return
+		}
+		c.JSON(http.StatusConflict, gin.H{"error": "domain already exists with status: " + existing.Status})
 		return
 	}