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/astro.config.mjs

@@ -1,5 +1,6 @@
 import { defineConfig } from 'astro/config';
 import vercel from '@astrojs/vercel';
+import db from '@astrojs/db';
 import auth from 'auth-astro';
 import remarkDirective from 'remark-directive';
 import remarkGfm from 'remark-gfm';
@@ -10,7 +11,7 @@ import remarkAside from './src/plugins/remark-aside.ts';
 export default defineConfig({
   output: 'static',
   adapter: vercel(),
-  integrations: [auth()],
+  integrations: [db(), auth()],
   markdown: {
     remarkPlugins: [
       remarkGfm,