Add GEO metadata: OG tags, JSON-LD, canonical, favicon, robots, sitemap, llms.txt

[CarmenDou]

Summary

Fixes every issue flagged in the GEO report for https://aipdfchat.insforge.site (GEO health 65/100, 2 urgent + 5 to-tune findings, AI footprint 0/100). All seven items land through standard Next.js App Router conventions, and every absolute URL derives from NEXT_PUBLIC_BETTER_AUTH_URL so template forks get correct tags on their own domain with zero extra config.

Stacked on #56 (base: fix/rag-recall-guidance). Will retarget to main automatically once #56 merges.

GEO finding	Fix
Open Graph tags 0/3 (urgent)	openGraph block in app/layout.tsx + app/opengraph-image.png file convention
No JSON-LD (urgent)	schema.org SoftwareApplication script in the root layout
No canonical	metadataBase + alternates.canonical
No twitter:card	twitter.card: summary_large_image (image auto-wired from the OG file convention)
No favicon	app/icon.png (512x512, generated brand logo)
No llms.txt	public/llms.txt markdown brief for AI engines
No sitemap in robots	app/robots.ts + app/sitemap.ts (public pages only; /api/ and unlisted /share/ links disallowed)

Test plan

• npm run build — /icon.png, /opengraph-image.png, /robots.txt, /sitemap.xml all appear in the route list
• curl /robots.txt shows sitemap reference and /share/ disallow
• View page source: og:, twitter:, canonical, and JSON-LD script present
• Favicon renders in browser tab
• Share the URL in Slack/X — rich card with logo renders

🤖 Generated with Claude Code

---

Summary by cubic

Adds full GEO/SEO metadata to improve link previews, search indexing, and LLM discoverability. Uses the final brand logo for the favicon and OG preview. All absolute URLs are built from NEXT_PUBLIC_BETTER_AUTH_URL.

• New Features

  • OG + Twitter cards and canonical tags in app/layout.tsx; preview image via app/opengraph-image.png (final brand logo).
  • JSON-LD SoftwareApplication script injected in the root layout.
  • Favicon via app/icon.png (final brand logo).
  • robots.txt and sitemap.xml via app/robots.ts and app/sitemap.ts; public pages only; disallow /api/ and /share/.
  • Added public/llms.txt with a concise product brief.

• Migration

  • Set NEXT_PUBLIC_BETTER_AUTH_URL to your public base URL in all environments.

^{Written for commit 04a92ff. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}

[jwfing]

LGTM - approved.