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/handler/ops_error_logger.go

@@ -423,6 +423,27 @@ func OpsErrorLoggerMiddleware(ops *service.OpsService) gin.HandlerFunc {
 					}
 				}
 			}
+			if v, ok := c.Get(service.OpsUpstreamErrorsKey); ok {
+				if events, ok := v.([]*service.OpsUpstreamErrorEvent); ok && len(events) > 0 {
+					entry.UpstreamErrors = events
+					// Best-effort backfill the single upstream fields from the last event when missing.
+					last := events[len(events)-1]
+					if last != nil {
+						if entry.UpstreamStatusCode == nil && last.UpstreamStatusCode > 0 {
+							code := last.UpstreamStatusCode
+							entry.UpstreamStatusCode = &code
+						}
+						if entry.UpstreamErrorMessage == nil && strings.TrimSpace(last.Message) != "" {
+							msg := strings.TrimSpace(last.Message)
+							entry.UpstreamErrorMessage = &msg
+						}
+						if entry.UpstreamErrorDetail == nil && strings.TrimSpace(last.Detail) != "" {
+							detail := strings.TrimSpace(last.Detail)
+							entry.UpstreamErrorDetail = &detail
+						}
+					}
+				}
+			}
 		}
 
 		if apiKey != nil {