ops: preview deployments for every pull request

[joelpeace48-cell]

Problem

Reviewing a UI change currently means checking out the branch and running it
locally. That is friction for maintainers and a genuine barrier for outside
contributors, who cannot easily show what their change looks like.

What to do

• Preview deployment per pull request, with the URL posted as a comment.
• Preview builds must point at testnet and staging services, never production —
  and must be visibly badged as previews.
• Block indexing on preview domains (see the robots issue) so they do not
  compete with production in search results.
• Run Lighthouse against the preview and post the scores, so performance and
  accessibility regressions are visible in review.
• Expire previews on merge or close so they do not accumulate.

Acceptance criteria

• Preview per pull request with the URL commented
• Previews point at testnet and staging only
• Preview domains not indexed
• Lighthouse scores posted to the pull request
• Previews cleaned up automatically

Notes

Posting Lighthouse scores in the pull request is what makes the accessibility
and performance work stick — otherwise regressions are only caught when someone
remembers to audit.