feat: IAM impersonation support — isImpersonated flag + banner

- src/auth.js: fetchIamUser extract isImpersonated from response
- src/routes/oidc.js: save isImpersonated to session
- ui/index.js: isImpersonated in req.user
- ui/routes/entries.js: pass isImpersonated to render
- views/index.ejs: banner 'Режим имперсонации'
- docs/PLAN-impersonation.md: план дальнейших шагов (двойной аудит)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ipwhitelist",
-  "version": "0.5.51",
+  "version": "0.5.52",
   "description": "IP WhiteList microservice for cloud provider",
   "main": "server.js",
   "scripts": {