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.

package.json

@@ -11,6 +11,15 @@
   },
   "dependencies": {
     "astro": "^4.16.18",
-    "@astrojs/sitemap": "^3.2.2"
+    "@astrojs/sitemap": "^3.2.2",
+    "@astrojs/node": "^8.3.4",
+    "better-sqlite3": "^9.4.3",
+    "@simplewebauthn/server": "^9.0.3",
+    "@simplewebauthn/browser": "^9.0.1",
+    "uuid": "^9.0.1"
+  },
+  "devDependencies": {
+    "@types/better-sqlite3": "^7.6.10",
+    "@types/uuid": "^9.0.7"
   }
 }