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/loose_ends.rs

@@ -16,7 +16,7 @@
 use std::time::{SystemTime, UNIX_EPOCH};
 
 use anyhow::Result;
-use hive_sh4re::{MANAGER_AGENT, LooseEnd};
+use hive_sh4re::{LooseEnd, MANAGER_AGENT};
 
 use crate::coordinator::Coordinator;