Gate admin dashboard with shared portal password

[ethgr0wth]

Summary

• add a configurable PORTAL_ADMIN_PORTAL_PASSWORD setting and enforce the shared key on every admin API route
• gate the admin dashboard in the frontend behind an unlock form that stores the password client-side and forwards it with each admin request
• document the new password requirement and extend automated tests to cover the admin access dependency

Testing

• PYENV_VERSION=3.11.12 python -m pytest ambassadors/node-operator-rewards-portal/backend/tests/test_rewards.py

---

https://chatgpt.com/codex/tasks/task_e_68f195a2ef54833387184a412e589db3