feat(auth): restore Google sign-in and AI admin test routes

Google OAuth was implemented locally but never deployed; the login button
only renders when AUTH_GOOGLE_ID and AUTH_GOOGLE_SECRET are set. Also
restores /api/ai/test-* endpoints removed by mistake and wires Google
credentials into deploy workflows.

Co-authored-by: Cursor <cursoragent@cursor.com>

memento-note/app/(auth)/register/page.tsx

@@ -1,5 +1,6 @@
 import { RegisterForm } from '@/components/register-form';
 import { getSystemConfig } from '@/lib/config';
+import { isGoogleAuthEnabled } from '@/lib/auth-providers';
 import { redirect } from 'next/navigation';
 
 export default async function RegisterPage() {
@@ -10,5 +11,5 @@ export default async function RegisterPage() {
     redirect('/login');
   }
 
-  return <RegisterForm />;
+  return <RegisterForm googleAuthEnabled={isGoogleAuthEnabled()} />;
 }