feat(ai-proxy): Update 9router button — shut down, npm update, relaunch hidden

Settings → AI Proxy gains an admin-only Update 9router button beside
Config 9router. One in-memory job (server/lib/nine-router-update.js) stops
9router (its user unit, else this account's own 9router processes), runs
npm install -g --prefix <its prefix> 9router@latest (sudo -n only when
sudoers already allows it, probed before anything stops), then relaunches
it hidden only if it was running. A failed update still relaunches the old
version. Nothing in the request names a command, path or package.

server/lib/auth.js

@@ -80,6 +80,9 @@ const ADMIN_WRITE_PREFIXES = [
   "/accounts",
   // Alert rules and budgets fire for every user's sessions.
   "/alerts/rules",
+  // Stopping, updating and restarting the host's 9router (both mounts).
+  "/ai-proxies/9router",
+  "/ai-routers/9router",
 ];
 
 /**