Fix build script dropping insights thumbnails and article links

[chellej]

Summary

• scripts/generate-site.py's INSIGHTS_ARTICLES data never carried the per-article thumbnail image or destination slug that were hand-patched directly into site/insights.html in Use new insights thumbnails #48/Add story card thumbnail images to insights page #49. Any rebuild (make build, or the CI Generate site step) silently regenerated insights.html without the thumbnails and pointed every "Read" link at /contact.html instead of the actual report page.
• Fixed by adding slug, image, and image_alt fields to INSIGHTS_ARTICLES and rendering them in insights_main(), so the generator is the single source of truth again and a rebuild can no longer regress this content.
• Out-of-scope but adjacent, included per maintainer practice: the shared footer copy in the chrome template had already been corrected in the generator source but never baked into the committed HTML - this rebuild picks that up across every page, along with a same-day sitemap.xml lastmod refresh. Both are inherent side effects of running a full, correct rebuild.

Root cause

Content was patched into generated HTML output directly instead of into the generator's source data, so the two drifted silently. The fix moves the data back into the generator so site/*.html is once again fully reproducible from scripts/generate-site.py.

Test plan

• python3 scripts/generate-site.py runs clean, writes 37 files
• Diffed output against the pre-existing committed site/insights.html - thumbnails and article links now match exactly
• Ran the build a second time - zero diff (idempotent)
• Ran the CI link-checker's internal-link scan against the regenerated site/ - 0 missing links
• python3 -m py_compile scripts/generate-site.py