feat: 新增全局错误透传规则功能

支持管理员配置上游错误如何返回给客户端：
- 新增 ErrorPassthroughRule 数据模型和 Ent Schema
- 实现规则的 CRUD API（/admin/error-passthrough-rules）
- 支持按错误码、关键词匹配，支持 any/all 匹配模式
- 支持按平台过滤（anthropic/openai/gemini/antigravity）
- 支持透传或自定义响应状态码和错误消息
- 实现两级缓存（Redis + 本地内存）和多实例同步
- 集成到 gateway_handler 的错误处理流程
- 新增前端管理界面组件
- 新增单元测试覆盖核心匹配逻辑

优化：
- 移除 refreshLocalCache 中的冗余排序（数据库已排序）
- 后端 Validate() 增加匹配条件非空校验

backend/internal/handler/gemini_v1beta_handler.go

@@ -253,7 +253,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	maxAccountSwitches := h.maxAccountSwitchesGemini
 	switchCount := 0
 	failedAccountIDs := make(map[int64]struct{})
-	lastFailoverStatus := 0
+	var lastFailoverErr *service.UpstreamFailoverError
 
 	for {
 		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, failedAccountIDs, "") // Gemini 不使用会话限制
@@ -262,7 +262,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				googleError(c, http.StatusServiceUnavailable, "No available Gemini accounts: "+err.Error())
 				return
 			}
-			handleGeminiFailoverExhausted(c, lastFailoverStatus)
+			h.handleGeminiFailoverExhausted(c, lastFailoverErr)
 			return
 		}
 		account := selection.Account
@@ -353,11 +353,11 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 			if errors.As(err, &failoverErr) {
 				failedAccountIDs[account.ID] = struct{}{}
 				if switchCount >= maxAccountSwitches {
-					lastFailoverStatus = failoverErr.StatusCode
-					handleGeminiFailoverExhausted(c, lastFailoverStatus)
+					lastFailoverErr = failoverErr
+					h.handleGeminiFailoverExhausted(c, lastFailoverErr)
 					return
 				}
-				lastFailoverStatus = failoverErr.StatusCode
+				lastFailoverErr = failoverErr
 				switchCount++
 				log.Printf("Gemini account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
 				continue
@@ -414,7 +414,36 @@ func parseGeminiModelAction(rest string) (model string, action string, err error
 	return "", "", &pathParseError{"invalid model action path"}
 }
 
-func handleGeminiFailoverExhausted(c *gin.Context, statusCode int) {
+func (h *GatewayHandler) handleGeminiFailoverExhausted(c *gin.Context, failoverErr *service.UpstreamFailoverError) {
+	if failoverErr == nil {
+		googleError(c, http.StatusBadGateway, "Upstream request failed")
+		return
+	}
+
+	statusCode := failoverErr.StatusCode
+	responseBody := failoverErr.ResponseBody
+
+	// 先检查透传规则
+	if h.errorPassthroughService != nil && len(responseBody) > 0 {
+		if rule := h.errorPassthroughService.MatchRule(service.PlatformGemini, statusCode, responseBody); rule != nil {
+			// 确定响应状态码
+			respCode := statusCode
+			if !rule.PassthroughCode && rule.ResponseCode != nil {
+				respCode = *rule.ResponseCode
+			}
+
+			// 确定响应消息
+			msg := service.ExtractUpstreamErrorMessage(responseBody)
+			if !rule.PassthroughBody && rule.CustomMessage != nil {
+				msg = *rule.CustomMessage
+			}
+
+			googleError(c, respCode, msg)
+			return
+		}
+	}
+
+	// 使用默认的错误映射
 	status, message := mapGeminiUpstreamError(statusCode)
 	googleError(c, status, message)
 }