feat: bootstrap admin auth, DB schema, and encrypted API keys

- Add Prisma schema for AdminUser, Source, Article, SearchRule, SearchRun,
  and encrypted ApiKey storage.
- Implement JWT session middleware and admin login/logout.
- Add AES-256-GCM encryption helpers for API keys.
- Seed default news sources and admin credentials.
- Create admin shell layout and login page.

src/app/admin/layout.tsx

@@ -0,0 +1,5 @@
+import AdminLayout from '@/components/admin/AdminLayout';
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return <AdminLayout>{children}</AdminLayout>;
+}