feat(ops): 实现上游错误事件记录与查询功能

**新增功能**:
- 新建ops_upstream_error_events表存储上游服务错误详情
- 支持记录上游429/529/5xx错误的详细上下文信息
- 提供按时间范围查询上游错误事件的API

**后端改动**:
1. 模型层（ops_models.go, ops_port.go）:
   - 新增UpstreamErrorEvent结构体
   - 扩展Repository接口支持上游错误事件CRUD

2. 仓储层（ops_repo.go）:
   - 实现InsertUpstreamErrorEvent写入上游错误
   - 实现GetUpstreamErrorEvents按时间范围查询

3. 服务层（ops_service.go, ops_upstream_context.go）:
   - ops_service: 新增GetUpstreamErrorEvents查询方法
   - ops_upstream_context: 封装上游错误上下文构建逻辑

4. Handler层（ops_error_logger.go）:
   - 新增GetUpstreamErrorsHandler处理上游错误查询请求

5. Gateway层集成:
   - antigravity_gateway_service.go: 429/529错误时记录上游事件
   - gateway_service.go: OpenAI 429/5xx错误时记录
   - gemini_messages_compat_service.go: Gemini 429/5xx错误时记录
   - openai_gateway_service.go: OpenAI 429/5xx错误时记录
   - ratelimit_service.go: 429限流错误时记录

**数据记录字段**:
- request_id: 关联ops_logs主记录
- platform/model: 上游服务标识
- status_code/error_message: 错误详情
- request_headers/response_body: 调试信息（可选）
- created_at: 错误发生时间

backend/internal/service/openai_gateway_service.go

@@ -511,7 +511,7 @@ func (s *OpenAIGatewayService) shouldFailoverUpstreamError(statusCode int) bool
 }
 
 func (s *OpenAIGatewayService) handleFailoverSideEffects(ctx context.Context, resp *http.Response, account *Account) {
-	body, _ := io.ReadAll(resp.Body)
+	body, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
 	s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, body)
 }
 
@@ -590,6 +590,13 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		// Ensure the client receives an error response (handlers assume Forward writes on non-failover errors).
 		safeErr := sanitizeUpstreamErrorMessage(err.Error())
 		setOpsUpstreamError(c, 0, safeErr, "")
+		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+			Platform:           account.Platform,
+			AccountID:          account.ID,
+			UpstreamStatusCode: 0,
+			Kind:               "request_error",
+			Message:            safeErr,
+		})
 		c.JSON(http.StatusBadGateway, gin.H{
 			"error": gin.H{
 				"type":    "upstream_error",
@@ -603,6 +610,30 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	// Handle error response
 	if resp.StatusCode >= 400 {
 		if s.shouldFailoverUpstreamError(resp.StatusCode) {
+			respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+			_ = resp.Body.Close()
+			resp.Body = io.NopCloser(bytes.NewReader(respBody))
+
+			upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
+			upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+			upstreamDetail := ""
+			if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
+				maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
+				if maxBytes <= 0 {
+					maxBytes = 2048
+				}
+				upstreamDetail = truncateString(string(respBody), maxBytes)
+			}
+			appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+				Platform:           account.Platform,
+				AccountID:          account.ID,
+				UpstreamStatusCode: resp.StatusCode,
+				UpstreamRequestID:  resp.Header.Get("x-request-id"),
+				Kind:               "failover",
+				Message:            upstreamMsg,
+				Detail:             upstreamDetail,
+			})
+
 			s.handleFailoverSideEffects(ctx, resp, account)
 			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode}
 		}
@@ -743,6 +774,15 @@ func (s *OpenAIGatewayService) handleErrorResponse(ctx context.Context, resp *ht
 
 	// Check custom error codes
 	if !account.ShouldHandleErrorCode(resp.StatusCode) {
+		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+			Platform:           account.Platform,
+			AccountID:          account.ID,
+			UpstreamStatusCode: resp.StatusCode,
+			UpstreamRequestID:  resp.Header.Get("x-request-id"),
+			Kind:               "http_error",
+			Message:            upstreamMsg,
+			Detail:             upstreamDetail,
+		})
 		c.JSON(http.StatusInternalServerError, gin.H{
 			"error": gin.H{
 				"type":    "upstream_error",
@@ -760,6 +800,19 @@ func (s *OpenAIGatewayService) handleErrorResponse(ctx context.Context, resp *ht
 	if s.rateLimitService != nil {
 		shouldDisable = s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, body)
 	}
+	kind := "http_error"
+	if shouldDisable {
+		kind = "failover"
+	}
+	appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+		Platform:           account.Platform,
+		AccountID:          account.ID,
+		UpstreamStatusCode: resp.StatusCode,
+		UpstreamRequestID:  resp.Header.Get("x-request-id"),
+		Kind:               kind,
+		Message:            upstreamMsg,
+		Detail:             upstreamDetail,
+	})
 	if shouldDisable {
 		return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode}
 	}