The admin was one shared HTTP Basic login: no way to give someone
partial access, revoke one person, or tell who changed what. Add a users
table and a session login form. Passwords are scrypt hashes from Ruby's
OpenSSL, so no native gem is needed. Sessions carry a per-user version
that is bumped on password change, which logs that user out everywhere.
ADMIN_USER/ADMIN_PASSWORD now bootstrap the first superuser on an empty
table and afterwards only work while no active superuser exists, as a
recovery path that re-creates one. Until per-area permissions land,
non-superusers only reach the dashboard and their own password page.
Assisted-by: Claude:opus-5
