fix(database): 计费巡检 - 清理未使用依赖并恢复后端构建

backend/internal/service/billing_review.go

@@ -3,8 +3,6 @@ package service
 import (
 	"log"
 	"time"
-
-	"infinite-canvas/backend/internal/repository"
 )
 
 const billingReviewStaleAfter = 15 * time.Minute