fix: improve SEO with JSON-LD, realistic sitemap lastmod, and cleanup

[majinghe]

Summary

Implements several SEO improvements identified in the analysis report:

1. JSON-LD Structured Data (P1)

• Homepage: Added Organization schema with name, url, logo, sameAs (GitHub, Twitter, Discord), contactPoint
• Blog posts: Added BlogPosting schema with headline, description, author, dates, publisher, mainEntityOfPage

Structured data helps search engines understand content and enables rich snippets in search results. Also improves visibility for AI-powered search engines (AI Overview, LLM citations).

2. Sitemap lastmod with Real Git Dates (P1)

• Modified scripts/generate-sitemap.js to query git log for each page's source file
• Blog posts now use their actual commit dates instead of build time
• Falls back to build time for untracked/generated pages
• Google explicitly states it ignores untrustworthy lastmod values; real git dates make the sitemap credible

3. Remove Meta Keywords (P2)

• Removed keywords field from homepage metadata
• Google has officially ignored meta keywords since 2009
• Keeping it only exposes keyword strategy to competitors

4. Fix Empty alt Text (P1)

• Featured blog post image on homepage: changed from alt="" to alt={post.title}
• Improves accessibility and enables image search indexing

Verification

• npx tsc --noEmit passes
• Clean build passes
• Sitemap lastmod now shows diverse dates (git commit times) instead of uniform build timestamp
• Homepage and blog posts contain JSON-LD blocks
• No lint errors introduced

Impact

These changes address the remaining P1/P2 SEO issues from the analysis report. Combined with PR #121 (docs 301 redirect) and PR #122 (canonical URLs), the site now has:

• ✅ Proper canonical URLs on all pages
• ✅ Server-side 301 for /docs (not JS redirect)
• ✅ JSON-LD structured data for rich snippets
• ✅ Credible sitemap lastmod values
• ✅ No obsolete meta keywords
• ✅ Image alt text for accessibility

Still TODO in future PRs:

• Add og:image/twitter:image (requires design assets)
• Increase internal link density (requires content planning)
• Performance optimization (Core Web Vitals baseline)

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