feat(seo): technical foundation - structured data, OG cards, bundle diet, FAQ - #20

What

Workstream A of the SEO plan (owner-approved; zero product/UI behavior change): structured data across docs, per-guide social cards, a 562KB bundle reduction, and a new landing FAQ section with matching FAQPage schema.

Why

Post-routes-restructure (#19) SEO foundation: rich results eligibility (FAQ/breadcrumbs/articles), better share CTR via branded cards, and page-speed as a ranking signal. FAQ added after owner sign-off on the one visible exception.

How

Four atomic commits:

1. Structured data - TechArticle + BreadcrumbList JSON-LD on all 20 doc routes; SoftwareApplication schema enriched with featureList.
2. Bundle diet - full-Prism imports replaced with PrismLight registering only used languages (docs + IntegrationShowcase): 1858KB -> 1296KB total JS (-562KB, -30%), killing the 765KB refractor chunk. Identical highlighting output.
3. Dynamic OG cards - opengraph-image.tsx prerenders a branded 1200x630 card per guide at build (title, category chip, wordmark); metadata auto-wires them, replacing generic /og.png for doc shares.
4. Landing FAQ - 6 engineer-facing questions as accessible accordions; visible answers and FAQPage JSON-LD render from one shared constant so they can never diverge.

Testing

• pnpm build clean (46 static pages incl. 20 OG cards); FAQPage + Breadcrumb JSON-LD verified in emitted HTML
• Highlighting visually unchanged; dev server reviewed live
• Product untouched: make lint pass, 300 tests green

Checklist

• Exactly one logical change in this PR (approved workstream A)
• make lint passes
• uv run python -m pytest is green (300 passed)
• Docs updated if user-facing
• CHANGELOG entry - n/a (site only)
• Conventional title