polish the api route, response, health, cache, and metrics

app/controllers/MetricsLite.php

@@ -0,0 +1,33 @@
+<?php
+require_once __DIR__.'/../helpers/json.php';
+
+// DB quick check
+$db_ok = null;
+try {
+  require_once __DIR__.'/../db.php';
+  $pdo = getPDO();
+  $pdo->query('SELECT 1');
+  $db_ok = true;
+} catch (Throwable $e) {
+  $db_ok = false;
+}
+
+// Rounded loadavg as STRINGS to avoid jq float parsing edge cases
+$loadavg_raw = function_exists('sys_getloadavg') ? sys_getloadavg() : null;
+$loadavg = null;
+if (is_array($loadavg_raw)) {
+  $loadavg = array_map(function($v){
+    return number_format((float)$v, 2, '.', ''); // string "6.03"
+  }, $loadavg_raw);
+}
+
+json_ok([
+  'time' => gmdate('c'),
+  'php_version' => PHP_VERSION,
+  'memory_used_bytes' => memory_get_usage(true),
+  'db' => $db_ok ? 'ok' : 'down',
+  'system' => [
+    'loadavg' => $loadavg,
+    'uptime' => @trim(shell_exec('uptime')),
+  ],
+]);