feat(auth): add NextAuth.js with credentials provider

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/app/api/auth/[...nextauth]/route.ts

@@ -0,0 +1,6 @@
+import NextAuth from 'next-auth';
+import { authOptions } from '@/lib/auth';
+
+const handler = NextAuth(authOptions);
+
+export { handler as GET, handler as POST };