feat(gateway): hivectl gateway user management + fix htpasswdFile assertion

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.

hive-c0re/src/questions.rs

@@ -236,8 +236,7 @@ mod tests {
         // exact MANAGER_AGENT constant passes).
         assert!(check_approval_canceller_is_manager("").is_err());
         assert!(
-            check_approval_canceller_is_manager(hive_sh4re::OPERATOR_RECIPIENT)
-                .is_err(),
+            check_approval_canceller_is_manager(hive_sh4re::OPERATOR_RECIPIENT).is_err(),
             "operator surface uses the dashboard cancel path, not this dispatcher",
         );
     }