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/Cargo.toml

@@ -9,6 +9,7 @@ workspace = true
 anyhow.workspace = true
 axum.workspace = true
 base64.workspace = true
+bcrypt.workspace = true
 reqwest.workspace = true
 clap.workspace = true
 hive-sh4re.workspace = true