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_port.go

@@ -81,6 +81,12 @@ type OpsInsertErrorLogInput struct {
 	UpstreamStatusCode   *int
 	UpstreamErrorMessage *string
 	UpstreamErrorDetail  *string
+	// UpstreamErrors captures all upstream error attempts observed during handling this request.
+	// It is populated during request processing (gin context) and sanitized+serialized by OpsService.
+	UpstreamErrors []*OpsUpstreamErrorEvent
+	// UpstreamErrorsJSON is the sanitized JSON string stored into ops_error_logs.upstream_errors.
+	// It is set by OpsService.RecordError before persisting.
+	UpstreamErrorsJSON *string
 
 	DurationMs         *int
 	TimeToFirstTokenMs *int64