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/repository/account_repo.go

@@ -80,6 +80,10 @@ func (r *accountRepository) Create(ctx context.Context, account *service.Account
 		SetSchedulable(account.Schedulable).
 		SetAutoPauseOnExpired(account.AutoPauseOnExpired)
 
+	if account.RateMultiplier != nil {
+		builder.SetRateMultiplier(*account.RateMultiplier)
+	}
+
 	if account.ProxyID != nil {
 		builder.SetProxyID(*account.ProxyID)
 	}
@@ -291,6 +295,10 @@ func (r *accountRepository) Update(ctx context.Context, account *service.Account
 		SetSchedulable(account.Schedulable).
 		SetAutoPauseOnExpired(account.AutoPauseOnExpired)
 
+	if account.RateMultiplier != nil {
+		builder.SetRateMultiplier(*account.RateMultiplier)
+	}
+
 	if account.ProxyID != nil {
 		builder.SetProxyID(*account.ProxyID)
 	} else {
@@ -999,6 +1007,11 @@ func (r *accountRepository) BulkUpdate(ctx context.Context, ids []int64, updates
 		args = append(args, *updates.Priority)
 		idx++
 	}
+	if updates.RateMultiplier != nil {
+		setClauses = append(setClauses, "rate_multiplier = $"+itoa(idx))
+		args = append(args, *updates.RateMultiplier)
+		idx++
+	}
 	if updates.Status != nil {
 		setClauses = append(setClauses, "status = $"+itoa(idx))
 		args = append(args, *updates.Status)
@@ -1347,6 +1360,8 @@ func accountEntityToService(m *dbent.Account) *service.Account {
 		return nil
 	}
 
+	rateMultiplier := m.RateMultiplier
+
 	return &service.Account{
 		ID:                  m.ID,
 		Name:                m.Name,
@@ -1358,6 +1373,7 @@ func accountEntityToService(m *dbent.Account) *service.Account {
 		ProxyID:             m.ProxyID,
 		Concurrency:         m.Concurrency,
 		Priority:            m.Priority,
+		RateMultiplier:      &rateMultiplier,
 		Status:              m.Status,
 		ErrorMessage:        derefString(m.ErrorMessage),
 		LastUsedAt:          m.LastUsedAt,