use middleware for runtime backend proxy

web/middleware.js

@@ -0,0 +1,11 @@
+import { NextResponse } from 'next/server'
+
+export function middleware(request) {
+  const backendUrl = process.env.BACKEND_URL || 'http://localhost:8080'
+  const { pathname, search } = request.nextUrl
+  return NextResponse.rewrite(new URL(pathname + search, backendUrl))
+}
+
+export const config = {
+  matcher: '/api/:path*',
+}