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/ent/usagelog/usagelog.go

@@ -54,6 +54,8 @@ const (
 	FieldActualCost = "actual_cost"
 	// FieldRateMultiplier holds the string denoting the rate_multiplier field in the database.
 	FieldRateMultiplier = "rate_multiplier"
+	// FieldAccountRateMultiplier holds the string denoting the account_rate_multiplier field in the database.
+	FieldAccountRateMultiplier = "account_rate_multiplier"
 	// FieldBillingType holds the string denoting the billing_type field in the database.
 	FieldBillingType = "billing_type"
 	// FieldStream holds the string denoting the stream field in the database.
@@ -144,6 +146,7 @@ var Columns = []string{
 	FieldTotalCost,
 	FieldActualCost,
 	FieldRateMultiplier,
+	FieldAccountRateMultiplier,
 	FieldBillingType,
 	FieldStream,
 	FieldDurationMs,
@@ -320,6 +323,11 @@ func ByRateMultiplier(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldRateMultiplier, opts...).ToFunc()
 }
 
+// ByAccountRateMultiplier orders the results by the account_rate_multiplier field.
+func ByAccountRateMultiplier(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldAccountRateMultiplier, opts...).ToFunc()
+}
+
 // ByBillingType orders the results by the billing_type field.
 func ByBillingType(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldBillingType, opts...).ToFunc()