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.

server/src/app.ts

@@ -73,7 +73,7 @@ export async function buildApp() {
 
   fastify.get('/api/health', async () => ({
     status: 'ok',
-    version: '0.3.0',
+    version: '0.3.1',
     timestamp: new Date().toISOString(),
   }))