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/config.ts

@@ -0,0 +1,16 @@
+import { defineDb, defineTable, column } from 'astro:db';
+
+const Guestbook = defineTable({
+  columns: {
+    id: column.number({ primaryKey: true }),
+    name: column.text(),
+    message: column.text(),
+    url: column.text({ optional: true }),
+    createdAt: column.date({ default: new Date() }),
+    approved: column.boolean({ default: false }),
+  },
+});
+
+export default defineDb({
+  tables: { Guestbook },
+});