Seo optimisation - fix redirects

[mneveroff]

Summary

Hotfix for a redirect loop on /lens/ introduced in #33.

After the SEO redirect work shipped, https://weeforest.org/lens/ returned ERR_TOO_MANY_REDIRECTS. The root cause was an app.get('/lens/', …) handler that called next() on clean requests; the Astro static fallback (extensions: ['html']) then handled the same path and issued another 301 → /lens/, looping forever.

This PR replaces the Lens app.get routes with app.use middleware that only redirects when it should:

• /lens → /lens/ (trailing slash)
• /lens/?ref=… (and other query variants) → /lens/ (strip tracking params)

Clean GET /lens/ requests pass through to Lens static and return 200.

Context

#33 added Ghost-era URL cleanup and SEO improvements after the Astro migration:

• 301s for /home/, /author/*, and /sitemap.xml
• robots.txt with sitemap-index.xml
• Lens canonical tag and /lens/ in the Astro sitemap
• Explicit robots.txt route from the Astro dist

Those changes remain on main; this PR only fixes the Lens redirect regression.

Test plan

• curl -sI http://localhost:3939/lens/ → 200
• curl -sI 'http://localhost:3939/lens?ref=test' → 301 → /lens/
• curl -sI 'http://localhost:3939/lens/?ref=test' → 301 → /lens/
• Browser: http://localhost:3939/lens/ loads the map
• After deploy: curl -sI https://weeforest.org/lens/ → 200
• After deploy: curl -sI https://weeforest.org/home/ → 301 → /
• Resubmit https://weeforest.org/sitemap-index.xml in Google Search Console

Post-deploy (manual)

1. Verify production /lens/ loads (not redirect loop).
2. In GSC: add sitemap-index.xml, remove stale sitemap.xml entry if still present.
3. If Cloudflare still serves its managed robots.txt, disable that so origin robots.txt is used.