fix(website): serve 410 for removed legacy URLs

[denhamparry]

Summary

Addresses part of #290 (Google Search Console index coverage findings).

Old blog content (/posts/*, /page/*, /cv/*) was permanently removed in a previous cleanup, but _redirects 301-redirected every one of those URLs to the homepage. Google treats bulk redirects-to-homepage as soft 404s, which is the likely source of the "Page with redirect" (3) and "Crawled – currently not indexed" (3) findings in the Search Console export.

This PR replaces those homepage redirects with 410 Gone responses (served via the generated 404.html body), the cleanest signal that the content is permanently gone so Google drops the URLs from its index.

Changes

• _redirects: legacy content paths now return 410 instead of 301 → /
• Canonical domain consolidation redirects (*.netlify.com, blog.* → denhamparry.co.uk) are unchanged

-https://denhamparry.co.uk/posts/* https://denhamparry.co.uk/:splat 301!
-https://denhamparry.co.uk/page/*  https://denhamparry.co.uk/:splat 301!
-https://denhamparry.co.uk/cv/*    https://denhamparry.co.uk/:splat 301!
+/posts/* /404.html 410
+/page/*  /404.html 410
+/cv/*    /404.html 410

Follow-up (tracked in #290)

This fixes the redirect strategy. The remaining tasks still need the specific URLs from Search Console's Pages report:

• Identify the single "Not found (404)" URL and confirm intent
• Review the 3 "currently not indexed" URLs once the redirect change has propagated
• Run "Validate Fix" in Search Console after deploy
• Re-export and refresh google_analytics/ to confirm improvement

Verification

• layouts/404.html exists, so Hugo emits /404.html for the 410 rule to serve
• pre-commit hooks pass (gitleaks, cspell, prettier)

🤖 Generated with Claude Code