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/client.rs

@@ -57,10 +57,7 @@ pub async fn build_and_restore(
         .trim()
         .to_owned();
     if token.is_empty() {
-        return Err(anyhow!(
-            "matrix token at {} is empty",
-            token_file.display()
-        ));
+        return Err(anyhow!("matrix token at {} is empty", token_file.display()));
     }
 
     let (user_id, device_id) = whoami(homeserver, &token).await?;