feat: switch to Astro DB for guestbook

- Add @astrojs/db integration
- Define Guestbook schema in db/config.ts
- Add seed data for development
- Update db.ts to use astro:db
- Add guestbook section to homepage with form
- Update env vars to use ASTRO_DB_REMOTE_URL

apps/blog/package.json

@@ -7,9 +7,9 @@
     "preview": "astro preview"
   },
   "dependencies": {
+    "@astrojs/db": "^0.19.0",
     "@astrojs/vercel": "^9.0.4",
     "@auth/core": "^0.37.4",
-    "@libsql/client": "^0.17.0",
     "astro": "^5.16.13",
     "auth-astro": "^4.2.0",
     "js-yaml": "^4.1.1"