chore(注释): 调整仪表盘注释为中文

backend/internal/service/dashboard_service.go

@@ -34,7 +34,7 @@ type dashboardStatsCacheEntry struct {
 	UpdatedAt int64                      `json:"updated_at"`
 }
 
-// DashboardService provides aggregated statistics for admin dashboard.
+// DashboardService 提供管理员仪表盘统计服务。
 type DashboardService struct {
 	usageRepo      UsageLogRepository
 	aggRepo        DashboardAggregationRepository