Add user domain verification that support subdomains

I want to verify `example.com` on my account and be able to add `hello.example.com` in a project without having to reverify it.

No files, tests, or entry points are named. Start by locating the account domain-verification and project custom-domain flows, then determine how verification is stored and checked. Done means a verified example.com account can add hello.example.com without a second verification.

Written by the indexing model from the issue text.