Remove the custom htpasswdFile option and bind-mount. The htpasswd file
now lives at the fixed path /var/lib/hyperhive/gateway/gateway.htpasswd
on the host, which is already exposed inside the container at
/run/hive-state/gateway.htpasswd via the existing gateway state
bind-mount — no extra bind-mount needed.
A tmpfiles rule pre-creates the file so nginx can open it even before
any users exist (empty file → all requests return 401, which is correct).
hivectl gateway commands default --file to the standard path so
`hivectl gateway create-user alice` just works without any flags.
Add `hivectl gateway {create-user,delete-user,list-users}` subcommands for
managing htpasswd files used by gateway Basic auth. Pure Rust bcrypt
(cost 12, $2y$ prefix nginx accepts). No external htpasswd binary required.
Also fix the NixOS module assertion: `cfg.auth ? htpasswdFile` is always
true in the module system (declared options always exist as keys); switch
to `nullOr path; default = null` + `!= null` check so the assertion
actually fires with a useful error when enable=true but no file is set.
Guard bind-mount and nginx config against null to prevent eval errors.
Update docs/gateway.md to show hivectl commands instead of raw htpasswd.
