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_models.go

@@ -36,12 +36,18 @@ type OpsErrorLogDetail struct {
 	ErrorBody string `json:"error_body"`
 	UserAgent string `json:"user_agent"`
 
+	// Upstream context (optional)
+	UpstreamStatusCode   *int   `json:"upstream_status_code,omitempty"`
+	UpstreamErrorMessage string `json:"upstream_error_message,omitempty"`
+	UpstreamErrorDetail  string `json:"upstream_error_detail,omitempty"`
+	UpstreamErrors       string `json:"upstream_errors,omitempty"` // JSON array (string) for display/parsing
+
 	// Timings (optional)
-	AuthLatencyMs       *int64 `json:"auth_latency_ms"`
-	RoutingLatencyMs    *int64 `json:"routing_latency_ms"`
-	UpstreamLatencyMs   *int64 `json:"upstream_latency_ms"`
-	ResponseLatencyMs   *int64 `json:"response_latency_ms"`
-	TimeToFirstTokenMs  *int64 `json:"time_to_first_token_ms"`
+	AuthLatencyMs      *int64 `json:"auth_latency_ms"`
+	RoutingLatencyMs   *int64 `json:"routing_latency_ms"`
+	UpstreamLatencyMs  *int64 `json:"upstream_latency_ms"`
+	ResponseLatencyMs  *int64 `json:"response_latency_ms"`
+	TimeToFirstTokenMs *int64 `json:"time_to_first_token_ms"`
 
 	// Retry context
 	RequestBody          string `json:"request_body"`
@@ -57,8 +63,8 @@ type OpsErrorLogFilter struct {
 	StartTime *time.Time
 	EndTime   *time.Time
 
-	Platform string
-	GroupID  *int64
+	Platform  string
+	GroupID   *int64
 	AccountID *int64
 
 	StatusCodes []int
@@ -71,9 +77,9 @@ type OpsErrorLogFilter struct {
 
 type OpsErrorLogList struct {
 	Errors   []*OpsErrorLog `json:"errors"`
-	Total    int           `json:"total"`
-	Page     int           `json:"page"`
-	PageSize int           `json:"page_size"`
+	Total    int            `json:"total"`
+	Page     int            `json:"page"`
+	PageSize int            `json:"page_size"`
 }
 
 type OpsRetryAttempt struct {
@@ -97,18 +103,18 @@ type OpsRetryAttempt struct {
 }
 
 type OpsRetryResult struct {
-	AttemptID int64 `json:"attempt_id"`
+	AttemptID int64  `json:"attempt_id"`
 	Mode      string `json:"mode"`
 	Status    string `json:"status"`
 
 	PinnedAccountID *int64 `json:"pinned_account_id"`
 	UsedAccountID   *int64 `json:"used_account_id"`
 
-	HTTPStatusCode int    `json:"http_status_code"`
+	HTTPStatusCode    int    `json:"http_status_code"`
 	UpstreamRequestID string `json:"upstream_request_id"`
 
-	ResponsePreview     string `json:"response_preview"`
-	ResponseTruncated   bool   `json:"response_truncated"`
+	ResponsePreview   string `json:"response_preview"`
+	ResponseTruncated bool   `json:"response_truncated"`
 
 	ErrorMessage string `json:"error_message"`