SEO polish: OG image, favicons, 404, article schema, image optimization

- Add branded 1200x630 og-default.jpg; article pages use their hero as
  og:image, others use the default
- Wire the existing (previously unlinked) favicon set into every page
- Add branded 404.html with root-absolute paths + noindex
- Emit Article + localized BreadcrumbList JSON-LD on all article pages
- Convert 74 heavy served photos to WebP (28MB -> 8.5MB, -70%), rewrite
  references, add loading=lazy/decoding=async to below-fold images and
  fetchpriority=high to heroes (protect LCP)
- Stop tracking images/original/ (458MB of unused source masters; kept on
  disk, gitignored) and ignore .playwright-mcp/

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

.gitignore

@@ -1 +1,3 @@
-.venv/
+.venv/
+images/original/
+.playwright-mcp/