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.

.gitignore

@@ -62,3 +62,9 @@ Thumbs.db
 !.env.example
 *.pem
 *.key
+
+# ---- Guestbook SQLite database (use Docker volume in production) ----
+data/
+*.db
+*.db-wal
+*.db-shm