feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

backend/internal/handler/dto/mappers.go

@@ -125,6 +125,7 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		ProxyID:                 a.ProxyID,
 		Concurrency:             a.Concurrency,
 		Priority:                a.Priority,
+		RateMultiplier:          a.BillingRateMultiplier(),
 		Status:                  a.Status,
 		ErrorMessage:            a.ErrorMessage,
 		LastUsedAt:              a.LastUsedAt,
@@ -279,6 +280,7 @@ func usageLogFromServiceBase(l *service.UsageLog, account *AccountSummary, inclu
 		TotalCost:             l.TotalCost,
 		ActualCost:            l.ActualCost,
 		RateMultiplier:        l.RateMultiplier,
+		AccountRateMultiplier: l.AccountRateMultiplier,
 		BillingType:           l.BillingType,
 		Stream:                l.Stream,
 		DurationMs:            l.DurationMs,