Invites, part 2c: a new forge (spawned or fresh) starts with signup = invite; existing forges keep their value #2419

Part of #2336.

Done looks like:

• [ ] A fresh data directory and a spawned child both start with signup = invite. A forge whose setting is already stored keeps it, with no migration that rewrites it.
• [ ] The forge's first account is still made without an invite, as #2256 left it.
• [ ] Tests for both cases. Run every suite that signs people up (grep -l signup test/*.ts); a suite that assumed open signup sets signup = open in its own setup and changes no assertion.