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/repository/ops_repo.go

@@ -53,6 +53,7 @@ INSERT INTO ops_error_logs (
   upstream_status_code,
   upstream_error_message,
   upstream_error_detail,
+  upstream_errors,
   duration_ms,
   time_to_first_token_ms,
   request_body,
@@ -63,7 +64,7 @@ INSERT INTO ops_error_logs (
   retry_count,
   created_at
 ) VALUES (
-  $1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25,$26,$27,$28,$29,$30,$31,$32,$33
+  $1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25,$26,$27,$28,$29,$30,$31,$32,$33,$34
 ) RETURNING id`
 
 	var id int64
@@ -94,6 +95,7 @@ INSERT INTO ops_error_logs (
 		opsNullInt(input.UpstreamStatusCode),
 		opsNullString(input.UpstreamErrorMessage),
 		opsNullString(input.UpstreamErrorDetail),
+		opsNullString(input.UpstreamErrorsJSON),
 		opsNullInt(input.DurationMs),
 		opsNullInt64(input.TimeToFirstTokenMs),
 		opsNullString(input.RequestBodyJSON),
@@ -267,6 +269,10 @@ SELECT
   COALESCE(request_id, ''),
   COALESCE(error_message, ''),
   COALESCE(error_body, ''),
+  upstream_status_code,
+  COALESCE(upstream_error_message, ''),
+  COALESCE(upstream_error_detail, ''),
+  COALESCE(upstream_errors::text, ''),
   is_business_limited,
   user_id,
   api_key_id,
@@ -292,6 +298,7 @@ LIMIT 1`
 	var out service.OpsErrorLogDetail
 	var latency sql.NullInt64
 	var statusCode sql.NullInt64
+	var upstreamStatusCode sql.NullInt64
 	var clientIP sql.NullString
 	var userID sql.NullInt64
 	var apiKeyID sql.NullInt64
@@ -318,6 +325,10 @@ LIMIT 1`
 		&out.RequestID,
 		&out.Message,
 		&out.ErrorBody,
+		&upstreamStatusCode,
+		&out.UpstreamErrorMessage,
+		&out.UpstreamErrorDetail,
+		&out.UpstreamErrors,
 		&out.IsBusinessLimited,
 		&userID,
 		&apiKeyID,
@@ -350,6 +361,10 @@ LIMIT 1`
 		s := clientIP.String
 		out.ClientIP = &s
 	}
+	if upstreamStatusCode.Valid && upstreamStatusCode.Int64 > 0 {
+		v := int(upstreamStatusCode.Int64)
+		out.UpstreamStatusCode = &v
+	}
 	if userID.Valid {
 		v := userID.Int64
 		out.UserID = &v
@@ -401,6 +416,11 @@ LIMIT 1`
 	if out.RequestHeaders == "null" {
 		out.RequestHeaders = ""
 	}
+	// Normalize upstream_errors to empty string when stored as JSON null.
+	out.UpstreamErrors = strings.TrimSpace(out.UpstreamErrors)
+	if out.UpstreamErrors == "null" {
+		out.UpstreamErrors = ""
+	}
 
 	return &out, nil
 }