Hi Does Kratos have built-in HA (High Availability) features? I'm concerned about system stability and looked through the official documentation, but I couldn't find any information related to this. If there is, could you please share the relevant link? If not, could you advise on how to set it up? I'm wondering if I need to run multiple Kratos instances and use a load balancer like HAProxy to distribute the traffic.

Yes, Kratos supports high availability and you can run multiple instances behind a load balancer, connecting to the same database. According to the docs you should have only one instance of courier running though