build: stabilize image optimization against EAGAIN/EPIPE failures

[RaminNietzsche]

Summary

Fixes intermittent npm start and npm run build failures caused by
imagemin spawning too many pngquant / mozjpeg child processes at
once (EAGAIN on macOS, EPIPE on some JPEGs).

Problem

• scripts/images.js optimized all ~1400 images in a single imagemin()
  call and wrote outputs with un-awaited forEach(async …)
• npm run dev ran assets in parallel with Eleventy, CSS, and Rollup,
  increasing process spawn pressure
• A handful of event JPEGs fail mozjpeg with EPIPE; the whole build
  exited with code 1

Changes

scripts/images.js

• Process images in batches of 8
• On batch failure, retry file-by-file
• On single-file failure, copy the original to dist/ (with warning)
• Use fs/promises and path.join (no undeclared make-dir / graceful-fs)

package.json

• dev: run assets before --serve (not in parallel with Eleventy)
• build: run assets after parallel site css js

Related PRs

Overlaps with open #994 (imagemin EPIPE), but this change is scoped to
the asset pipeline only — no footer / Twitter icon changes.

[ceph-jenkins]

Thank you for your contribution. Since you, the author, are not a member of the Ceph GitHub Org yet, our CI will not automatically run. Any member of the Ceph Org may comment "ok - to - test" (without the dashes) to allow the Jenkins jobs to run.