Fix: Multi-user data isolation and Legacy migration logic

app/api/auth/logout/route.ts

@@ -0,0 +1,7 @@
+import { NextResponse } from 'next/server';
+import { destroySession } from '@/lib/auth';
+
+export async function POST() {
+  destroySession();
+  return NextResponse.json({ success: true });
+}