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.

.env.example

@@ -0,0 +1,24 @@
+# Required: random secret used to salt IP hashes and sign sessions
+# Generate with: openssl rand -hex 32
+SECRET_KEY=replace_me_with_a_random_secret
+
+# WebAuthn / YubiKey configuration
+# rpID must match the domain your site is served from (no scheme, no port)
+WEBAUTHN_RP_ID=hiddenden.cafe
+# Full origin including scheme (and port if non-standard)
+WEBAUTHN_ORIGIN=https://hiddenden.cafe
+# Human-readable name shown in the YubiKey prompt
+WEBAUTHN_RP_NAME=Cozy Den
+
+# Database path (Docker mounts /data as a named volume)
+DB_PATH=/data/guestbook.db
+
+# Server binding
+HOST=0.0.0.0
+PORT=3000
+
+# --- Development overrides ---
+# For local dev (npm run dev), override with:
+# WEBAUTHN_RP_ID=localhost
+# WEBAUTHN_ORIGIN=http://localhost:4321
+# DB_PATH=./data/guestbook.db