Feat: Host the NSA article's header image next to the article

Relative header_image paths (./img.webp) now resolve to /<slug>/img.webp, and article
assets also ship unhashed so og:image and JSON-LD URLs resolve.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/lib/renderers.mjs

@@ -98,7 +98,7 @@ export const renderArticle = (article, dates = articleDates(article)) => `
 ${shellHead({
   title: `${article.title} — apophenia.news`,
   desc: article.description,
-  image: article.header_image,
+  image: absUrl(article.header_image),
   url: `${SITE_URL}/${article.slug}/`,
   type: "article",
   ...dates,