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.

src/pages/api/admin/logout.ts

@@ -0,0 +1,14 @@
+import type { APIRoute } from 'astro';
+import { deleteSession, SESSION_COOKIE } from '../../../lib/auth';
+
+export const prerender = false;
+
+export const POST: APIRoute = async ({ cookies }) => {
+  const sessionId = cookies.get(SESSION_COOKIE)?.value;
+  if (sessionId) {
+    deleteSession(sessionId);
+    cookies.delete(SESSION_COOKIE, { path: '/' });
+  }
+
+  return new Response(null, { status: 303, headers: { Location: '/admin/login' } });
+};