Wire the existing allow_registration setting to an admin toggle in the
Users tab and a public /register page, so the documented self-registration
flow is reachable from the UI instead of the API alone. Expose the flag on
the public setup-status endpoint the router guard already calls, so the
sign-in page can offer sign-up without an extra request.
Harden the newly public path: return a conflict instead of a 500 when
concurrent signups race the unique index, sanitize the logged username,
and reject control, zero-width, and text-direction characters in the
username and display name so a new account cannot impersonate another in
the user list or audit log.
Closes#952
neon
e7039e93a0