Perf:
- AppShell: auth check once on mount instead of every pathname change
- Dashboard: replace bare <a> with Next Link for prefetch
Security:
- Migrate python-jose to PyJWT (CVE-2024-33663, CVE-2024-33664)
- JWT exp/iat now UTC-aware via datetime.now(timezone.utc)
- Drop default 'changeme' fallback for DEFAULT_ADMIN_PASSWORD
- Force POSTGRES_PASSWORD env in docker-compose