feat: add Next.js rewrite for /api → backend proxy

Routes /api/* requests to backend:8000 internally via Docker network.
Replaces nginx reverse proxy with Next.js built-in rewrites.

frontend/next.config.ts

@@ -4,6 +4,14 @@ const nextConfig: NextConfig = {
   turbopack: {
     root: __dirname,
   },
+  async rewrites() {
+    return [
+      {
+        source: "/api/:path*",
+        destination: "http://backend:8000/:path*",
+      },
+    ];
+  },
   images: {
     remotePatterns: [
       {