fix: api-proxy per middleware (laufzeit-env statt build-time rewrites)

frontend/middleware.ts

@@ -0,0 +1,15 @@
+import { NextResponse } from "next/server";
+import type { NextRequest } from "next/server";
+
+// Proxies /api/* to the backend. Runs per request, so BACKEND_URL is read
+// at RUNTIME (unlike next.config.js rewrites, which are baked at build time
+// in standalone mode and ignore container env vars).
+export function middleware(req: NextRequest) {
+  const backend = process.env.BACKEND_URL || "http://127.0.0.1:8000";
+  const target = new URL(req.nextUrl.pathname + req.nextUrl.search, backend);
+  return NextResponse.rewrite(target);
+}
+
+export const config = {
+  matcher: "/api/:path*",
+};

frontend/next.config.js

@@ -9,6 +9,8 @@ const nextConfig = {
     ],
   },
   async rewrites() {
+    // Dev fallback only. In Docker/standalone the middleware.ts proxy handles
+    // /api/* with the runtime BACKEND_URL (rewrites are build-time there).
     const backend = process.env.BACKEND_URL || "http://127.0.0.1:8000";
     return [
       { source: "/api/:path*", destination: `${backend}/api/:path*` },