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/ratelimit_service.go

@@ -55,19 +55,36 @@ func (s *RateLimitService) HandleUpstreamError(ctx context.Context, account *Acc
 	}
 
 	tempMatched := s.tryTempUnschedulable(ctx, account, statusCode, responseBody)
+	upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(responseBody))
+	upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+	if upstreamMsg != "" {
+		upstreamMsg = truncateForLog([]byte(upstreamMsg), 512)
+	}
 
 	switch statusCode {
 	case 401:
 		// 认证失败：停止调度，记录错误
-		s.handleAuthError(ctx, account, "Authentication failed (401): invalid or expired credentials")
+		msg := "Authentication failed (401): invalid or expired credentials"
+		if upstreamMsg != "" {
+			msg = "Authentication failed (401): " + upstreamMsg
+		}
+		s.handleAuthError(ctx, account, msg)
 		shouldDisable = true
 	case 402:
 		// 支付要求：余额不足或计费问题，停止调度
-		s.handleAuthError(ctx, account, "Payment required (402): insufficient balance or billing issue")
+		msg := "Payment required (402): insufficient balance or billing issue"
+		if upstreamMsg != "" {
+			msg = "Payment required (402): " + upstreamMsg
+		}
+		s.handleAuthError(ctx, account, msg)
 		shouldDisable = true
 	case 403:
 		// 禁止访问：停止调度，记录错误
-		s.handleAuthError(ctx, account, "Access forbidden (403): account may be suspended or lack permissions")
+		msg := "Access forbidden (403): account may be suspended or lack permissions"
+		if upstreamMsg != "" {
+			msg = "Access forbidden (403): " + upstreamMsg
+		}
+		s.handleAuthError(ctx, account, msg)
 		shouldDisable = true
 	case 429:
 		s.handle429(ctx, account, headers)