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/db/seed.ts

@@ -0,0 +1,22 @@
+import { db, Guestbook } from 'astro:db';
+
+export default async function seed() {
+  await db.insert(Guestbook).values([
+    {
+      id: 1,
+      name: 'alice',
+      message: 'love the site!',
+      url: 'https://example.com',
+      createdAt: new Date('2026-01-20'),
+      approved: true,
+    },
+    {
+      id: 2,
+      name: 'bob',
+      message: 'great blog posts',
+      url: null,
+      createdAt: new Date('2026-01-18'),
+      approved: true,
+    },
+  ]);
+}