fix: resolve EADDRINUSE port conflict — Node.js now listens on internal port 4000

Nginx listens publicly on 2087/2083 and reverse-proxies to 127.0.0.1:4000.
Previously both Nginx and Node.js were trying to bind port 2087 causing the
nixpanel service to crash-loop. HOST changed to 127.0.0.1 so the API is
not directly reachable from outside.

Bumps version to 0.3.1.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "nixpanel-server",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "private": true,
   "workspaces": [
     "server",