fix(frontend): proxy /api to Express in dev; harden media file serving

- next.config: development-only rewrite /api/:path* to 127.0.0.1:4000; move
  sharp to experimental.serverComponentsExternalPackages for Next 14
- media/[id]/route: resolve storage at request time, fallbacks for
  backend/storage/media vs storage/media, async params, force-dynamic

Made-with: Cursor

frontend/next.config.js

@@ -1,15 +1,24 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
-  serverExternalPackages: ['sharp'],
+  experimental: {
+    serverComponentsExternalPackages: ['sharp'],
+  },
   images: {
     remotePatterns: [
       { protocol: 'https', hostname: '**' },
     ],
   },
   async rewrites() {
-    return [
+    const rules = [
       { source: '/calendar.ics', destination: '/calendar' },
     ];
+    if (process.env.NODE_ENV === 'development') {
+      rules.push({
+        source: '/api/:path*',
+        destination: 'http://127.0.0.1:4000/api/:path*',
+      });
+    }
+    return rules;
   },
 };