perf: cache config.json reads by mtime, reduce sync I/O

backend/db.js

@@ -6,12 +6,17 @@ const crypto = require('crypto');
 const CONFIG_PATH = path.join(__dirname, '..', 'data', 'config.json');
 
 let pool = null;
+let _configCache = null;
+let _configMtime = 0;
 
 function getConfig() {
-  if (fs.existsSync(CONFIG_PATH)) {
-    return JSON.parse(fs.readFileSync(CONFIG_PATH, 'utf-8'));
-  }
-  return null;
+  try {
+    const stat = fs.statSync(CONFIG_PATH);
+    if (_configCache && stat.mtimeMs === _configMtime) return _configCache;
+    _configCache = JSON.parse(fs.readFileSync(CONFIG_PATH, 'utf-8'));
+    _configMtime = stat.mtimeMs;
+    return _configCache;
+  } catch { return null; }
 }
 
 function saveConfig(cfg) {