[User] Deter fake ages

I'm not saying you can't be 103 years old and using HCB, but i'm fairly certain there are some unreasonable ages in there (espeically given the peak at 100). In fact, there are 6 people who aren't born yet! oh!

We need this: https://www.tiktok.com/@thematchapost/video/7407859609320410410
and strong birthday/age validations in the User model.

Start by locating the User model and reading its existing birthday and age handling. Check the relevant User model tests, then define validation behavior for unreasonable and future birthdays based on the issue's examples. The work is done when invalid ages are rejected and the corresponding tests pass.

Written by the indexing model from the issue text.