Add article dates, brand OG image, launch SEO checklist

- Add datePublished/dateModified to Article JSON-LD on the 11 dated
  articles (curated once from EN, applied to all languages)
- Replace og-default.jpg with the official brand OG image (1200x630)
- Add docs/launch-seo-checklist.md (go-live steps: redirects, Search
  Console, analytics, rich-result validation, monitoring)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

en/projects/in-memory-of-frosina-kulakova/index.html

@@ -25,7 +25,7 @@
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