feat: add independent load_factor field for scheduling load calculation

backend/ent/schema/account.go

@@ -97,6 +97,8 @@ func (Account) Fields() []ent.Field {
 		field.Int("concurrency").
 			Default(3),
 
+		field.Int("load_factor").Optional().Nillable(),
+
 		// priority: 账户优先级，数值越小优先级越高
 		// 调度器会优先使用高优先级的账户
 		field.Int("priority").