Admin: split action handlers and refresh project docs

lib/admin_http.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+function adminJsonResponse(array $payload, int $statusCode = 200): never
+{
+    http_response_code($statusCode);
+    header('Content-Type: application/json; charset=utf-8');
+    echo json_encode($payload, JSON_UNESCAPED_UNICODE);
+    exit;
+}