feat: proxy backend API through Next.js using runtime BACKEND_URL

Previously, NEXT_PUBLIC_API_URL was a build-time Next.js variable, making
it impossible to configure the backend URL in docker-compose `environment`
without rebuilding the image.

This change introduces a proper server-side proxy:
- next.config.ts: adds a rewrite rule that forwards all /api/* requests
  to BACKEND_URL (read at server startup, not baked at build time).
  Defaults to http://localhost:8000 so local dev works without config.
- api.ts: API_BASE is now an empty string — all fetch calls use relative
  /api/... paths, which the Next.js server proxies to the backend.
- docker-compose.yml: replaces NEXT_PUBLIC_API_URL build arg with a
  runtime BACKEND_URL env var defaulting to http://backend:8000, using
  Docker's internal networking. Port 8000 no longer needs to be exposed.
- README: updates Docker setup docs, standalone compose example, and
  environment variable reference to reflect BACKEND_URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Former-commit-id: b4c9e78cdd

frontend/next.config.ts

@@ -1,5 +1,10 @@
 import type { NextConfig } from "next";
 
+// BACKEND_URL is a plain (non-NEXT_PUBLIC_) env var read at server startup —
+// not baked at build time — so it can be set in docker-compose `environment`.
+// Defaults to localhost for local dev where both services run on the same host.
+const backendUrl = process.env.BACKEND_URL ?? "http://localhost:8000";
+
 const nextConfig: NextConfig = {
   transpilePackages: ['react-map-gl', 'mapbox-gl', 'maplibre-gl'],
   output: "standalone",
@@ -9,6 +14,14 @@ const nextConfig: NextConfig = {
   eslint: {
     ignoreDuringBuilds: true,
   },
+  async rewrites() {
+    return [
+      {
+        source: "/api/:path*",
+        destination: `${backendUrl}/api/:path*`,
+      },
+    ];
+  },
 };
 
 export default nextConfig;