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/ops_upstream_context.go

@@ -1,7 +1,9 @@
 package service
 
 import (
+	"encoding/json"
 	"strings"
+	"time"
 
 	"github.com/gin-gonic/gin"
 )
@@ -12,6 +14,7 @@ const (
 	OpsUpstreamStatusCodeKey   = "ops_upstream_status_code"
 	OpsUpstreamErrorMessageKey = "ops_upstream_error_message"
 	OpsUpstreamErrorDetailKey  = "ops_upstream_error_detail"
+	OpsUpstreamErrorsKey       = "ops_upstream_errors"
 )
 
 func setOpsUpstreamError(c *gin.Context, upstreamStatusCode int, upstreamMessage, upstreamDetail string) {
@@ -29,3 +32,75 @@ func setOpsUpstreamError(c *gin.Context, upstreamStatusCode int, upstreamMessage
 	}
 }
 
+// OpsUpstreamErrorEvent describes one upstream error attempt during a single gateway request.
+// It is stored in ops_error_logs.upstream_errors as a JSON array.
+type OpsUpstreamErrorEvent struct {
+	AtUnixMs int64 `json:"at_unix_ms,omitempty"`
+
+	// Context
+	Platform  string `json:"platform,omitempty"`
+	AccountID int64  `json:"account_id,omitempty"`
+
+	// Outcome
+	UpstreamStatusCode int    `json:"upstream_status_code,omitempty"`
+	UpstreamRequestID  string `json:"upstream_request_id,omitempty"`
+
+	// Kind: http_error | request_error | retry_exhausted | failover
+	Kind string `json:"kind,omitempty"`
+
+	Message string `json:"message,omitempty"`
+	Detail  string `json:"detail,omitempty"`
+}
+
+func appendOpsUpstreamError(c *gin.Context, ev OpsUpstreamErrorEvent) {
+	if c == nil {
+		return
+	}
+	if ev.AtUnixMs <= 0 {
+		ev.AtUnixMs = time.Now().UnixMilli()
+	}
+	ev.Platform = strings.TrimSpace(ev.Platform)
+	ev.UpstreamRequestID = strings.TrimSpace(ev.UpstreamRequestID)
+	ev.Kind = strings.TrimSpace(ev.Kind)
+	ev.Message = strings.TrimSpace(ev.Message)
+	ev.Detail = strings.TrimSpace(ev.Detail)
+	if ev.Message != "" {
+		ev.Message = sanitizeUpstreamErrorMessage(ev.Message)
+	}
+
+	var existing []*OpsUpstreamErrorEvent
+	if v, ok := c.Get(OpsUpstreamErrorsKey); ok {
+		if arr, ok := v.([]*OpsUpstreamErrorEvent); ok {
+			existing = arr
+		}
+	}
+
+	evCopy := ev
+	existing = append(existing, &evCopy)
+	c.Set(OpsUpstreamErrorsKey, existing)
+}
+
+func getOpsUpstreamErrors(c *gin.Context) []*OpsUpstreamErrorEvent {
+	if c == nil {
+		return nil
+	}
+	if v, ok := c.Get(OpsUpstreamErrorsKey); ok {
+		if arr, ok := v.([]*OpsUpstreamErrorEvent); ok {
+			return arr
+		}
+	}
+	return nil
+}
+
+func marshalOpsUpstreamErrors(events []*OpsUpstreamErrorEvent) *string {
+	if len(events) == 0 {
+		return nil
+	}
+	// Ensure we always store a valid JSON value.
+	raw, err := json.Marshal(events)
+	if err != nil || len(raw) == 0 {
+		return nil
+	}
+	s := string(raw)
+	return &s
+}