feat(清理任务): 引入Ent存储并补充日志与测试

新增 usage_cleanup_task Ent schema 与仓储实现，支持清理任务排序分页
补充清理任务全链路日志、仪表盘重算触发及 UI 过滤调整
完善 repository/service 单测并引入 sqlite 测试依赖

backend/internal/handler/admin/usage_cleanup_handler_test.go

@@ -3,8 +3,8 @@ package admin
 import (
 	"bytes"
 	"context"
-	"encoding/json"
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"net/http"
 	"net/http/httptest"