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.
BindMount was imported at line 10 via the priv_proto glob-style import,
then re-exported again with `pub use` at line 69. E0252: two definitions
of BindMount in the same type namespace. Remove the redundant pub use —
callers that need BindMount should import from hive_sh4re::priv_proto.
