Kousa: Arch: Consider using Phoenix Tracker or Phoenix Presence instead of database

[ityonemo]

For keeping track of user online state. This information doesn't really need to live in the database, and having frequent read/write events to the db on this content will eventually hammer your database and cause responsiveness issues.

You don't need to pull in Phoenix as a dependency to use Phoenix Tracker, it exists as standalone library. https://hexdocs.pm/phoenix_pubsub/Phoenix.Tracker.html#content