I guarantee for nothing, since this is a spare time project. That said, I have had zero outages in 11 months, see https://ntfy.statuspage.io/
When it outgrows the single EC2 instance, I'll likely start building some distributed thingy to spread the load and do some HA things (sounds like fun). Probably just a load balancer and 2-3 instances with a rqlite backend instead of SQLite, nothing too fancy.
Right now that's not necessary. As you can see, it handles traffic just fine for now.