[SandObserver]

Summary

• Update the demo link in the README.md header to https://demo.sandobserver.com.
• Update the same link in the header scripts/dockerhub-description.js substitutes, which is the only demo link that reaches the Docker Hub page.
• Name demo.sandobserver.com as the subdomain the public demo is reached at in docs/demo.md, keeping the CNAME instruction unchanged.

Testing
All green, including the Docker build. No new tests; nothing pins either hostname.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.