Add guestbook with WebAuthn admin and SQLite

Introduce server-rendered guestbook and moderation portal.
Persist data in SQLite (better-sqlite3); add WebAuthn YubiKey
admin auth, rate-limiting, spam heuristics, and sanitization.
Switch Docker image to run Node/standalone Astro (remove nginx),
update docker-compose, Dockerfile, astro.config, and package.json.
Add .env.example, docs/guestbook.md, gitignore updates, layouts,
API routes, and supporting lib/components/pages for the feature.

astro.config.mjs

@@ -1,9 +1,14 @@
 import { defineConfig } from 'astro/config';
 import sitemap from '@astrojs/sitemap';
+import node from '@astrojs/node';
 
 // https://astro.build/config
 export default defineConfig({
   site: 'https://hiddenden.cafe',
+  output: 'hybrid',
+  adapter: node({
+    mode: 'standalone',
+  }),
   integrations: [
     sitemap({
       changefreq: 'weekly',