Better-Auth creates users via raw SQL INSERT (not through SQLAlchemy),
so it bypasses ORM defaults and causes HTTP 500 on sign-up/sign-in.
Adds PostgreSQL server_default so INSERT without email_inbound_token
auto-generates a URL-safe token matching Python secrets.token_urlsafe(16).
Co-Authored-By: Paperclip <noreply@paperclip.ing>