Fix: Append :path* to FASTAPI_URL in rewrites

The rewrite destination was using FASTAPI_URL directly, which
replaced the entire destination including the :path* parameter.
This caused /api/compare to rewrite to just http://backend:80/api
instead of http://backend:80/api/compare.

Now properly constructs: ${FASTAPI_URL}/:path*

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextjs-app/next.config.js

@@ -5,10 +5,11 @@ const nextConfig = {
 
   // API Proxy to FastAPI backend
   async rewrites() {
+    const apiUrl = process.env.FASTAPI_URL || 'http://localhost:8000/api';
     return [
       {
         source: '/api/:path*',
-        destination: process.env.FASTAPI_URL || 'http://localhost:8000/api/:path*',
+        destination: `${apiUrl}/:path*`,
       },
     ];
   },