Validating email is a CHECK constraint using a regexp on the email filed, resetting password is probably two short stored procedures. I get that you don't usually find code to copy/paste for something like this but it's not hard to implement either (i would say the whole signup/login/reset flow can be done in 100-150 LOC). Out of curiosity, how much time did it take to implement the API you needed?

A general rule of thumb is that Identity and Access Management aren't businesses you want to accidentally get into. Yes, you can write IAM code yourself, but at scale it's a way better plan to trust a dedicated IdP.