fix: 同步 LoadFactor 到调度快照缓存

LoadFactor 字段在构建调度快照缓存时缺失，导致调度算法
EffectiveLoadFactor() 无法使用配置的负载因子，回退到 Concurrency。
这会影响账号的负载率计算，进而影响调度优先级。

修复：在 buildSchedulerMetadataAccount 中添加 LoadFactor 字段。

backend/internal/repository/scheduler_cache.go

@@ -378,6 +378,7 @@ func buildSchedulerMetadataAccount(account service.Account) service.Account {
 		Platform:                account.Platform,
 		Type:                    account.Type,
 		Concurrency:             account.Concurrency,
+		LoadFactor:              account.LoadFactor,
 		Priority:                account.Priority,
 		RateMultiplier:          account.RateMultiplier,
 		Status:                  account.Status,