Add local development environment with switchable backend

Set up local dev workflow: SSH tunnel to dev DB, Next.js API proxy
via rewrites, Docker or native backend on port 5000. Update and
restructure project documentation.

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

frontend/next.config.ts

@@ -2,6 +2,16 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  async rewrites() {
+    const apiUrl = process.env.LOCAL_API_URL;
+    if (!apiUrl) return [];
+    return [
+      {
+        source: "/api/:path*",
+        destination: `${apiUrl}/api/:path*`,
+      },
+    ];
+  },
 };
 
 export default nextConfig;