fix(logging): 修复 warn 级别下系统日志空白问题

- 新增 logger.WriteSinkEvent，支持旁路写入 sink，不受全局级别门控影响\n- 在 http.access 中间件中，当 info 被门控时补写 sink，保障 Ops 系统日志可索引\n- 增加 level=warn 场景回归测试，验证访问日志仍可入库

backend/internal/server/middleware/logger.go

@@ -58,6 +58,36 @@ func Logger() gin.HandlerFunc {
 
 		l := logger.FromContext(c.Request.Context()).With(fields...)
 		l.Info("http request completed", zap.Time("completed_at", endTime))
+		// 当全局日志级别高于 info（如 warn/error）时，access info 不会进入 zap core，
+		// 这里补写一次 sink，保证 ops 系统日志仍可索引关键访问轨迹。
+		if !logger.L().Core().Enabled(logger.LevelInfo) {
+			sinkFields := map[string]any{
+				"component":    "http.access",
+				"status_code":  statusCode,
+				"latency_ms":   latency.Milliseconds(),
+				"client_ip":    clientIP,
+				"protocol":     protocol,
+				"method":       method,
+				"path":         path,
+				"completed_at": endTime,
+			}
+			if requestID, ok := c.Request.Context().Value(ctxkey.RequestID).(string); ok && requestID != "" {
+				sinkFields["request_id"] = requestID
+			}
+			if clientRequestID, ok := c.Request.Context().Value(ctxkey.ClientRequestID).(string); ok && clientRequestID != "" {
+				sinkFields["client_request_id"] = clientRequestID
+			}
+			if hasAccountID && accountID > 0 {
+				sinkFields["account_id"] = accountID
+			}
+			if platform != "" {
+				sinkFields["platform"] = platform
+			}
+			if model != "" {
+				sinkFields["model"] = model
+			}
+			logger.WriteSinkEvent("info", "http.access", "http request completed", sinkFields)
+		}
 
 		if len(c.Errors) > 0 {
 			l.Warn("http request contains gin errors", zap.String("errors", c.Errors.String()))