feat: Extract Developer Portal as standalone Next.js app on port 3006

SDK customers can now access the documentation publicly without login.
The portal runs independently from admin-v2 on https://macmini:3006/.

- New developer-portal/ app with 26 pages, 2 components
- Docker service + nginx SSL reverse proxy on port 3006
- All /developers/* routes remapped to /* in the new app
- admin-v2 developer pages remain unchanged

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

developer-portal/postcss.config.mjs

@@ -0,0 +1,9 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}
+
+export default config