Invites, part 2a: with signup = invite, /signup and POST /api/v1/users make an account only through a valid invite link #2417

Part of #2336. Try 1 timed out doing everything, and try 2 went red on test/tui-can.test.ts.

Done looks like:

• [ ] With signup = invite, the web form (?invite=<token>) and the API (invite) make an account only from a valid, unused, unexpired link (src/core/invites.ts), and use it up. Anything else is refused with one sentence that never says whether the token existed.
• [ ] Using a link goes in the audit. The terminal and the default for new forges are not part of this issue.
• [ ] Tests: the happy path; reuse, expiry and revocation each refused. Run test/signup.test.ts and test/tui-can.test.ts.