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-matrix-mcp/src/wake.rs

@@ -84,7 +84,10 @@ mod tests {
     #[test]
     fn format_wake_body_short_passes_through() {
         let body = format_wake_body("@iris:matrix.darkest.space", "#general", "hi all");
-        assert_eq!(body, "[matrix] @iris:matrix.darkest.space in #general: hi all");
+        assert_eq!(
+            body,
+            "[matrix] @iris:matrix.darkest.space in #general: hi all"
+        );
     }
 
     #[test]