Expose betterauth settings

What would you like to change?

Expose betterauth settings in .env or in web ui

Additional context

This would be nice because in self hosted use case, someone can have an email like name@mydomain.tld, but on github they will have name@gmail.com, betterauth doesnt like this by default and doesnt allow connecting github if the kaneo email and github primary email dont match [i think?]

---

Originally requested by @TadeasDitte on 2026-10-07. Original GitHub request #1957.