Phase 2 — Retire `/certified-nodes/`

[n-lark]

Phase 2 — Retire /certified-nodes/

Goal: Remove the duplicate page once /integrations/ covers its job.

• 301 redirect /certified-nodes/ → /integrations/?certified=1. Added to src/redirects.njk (the repo's path-redirect convention — generates a _redirects file served by Netlify). Covers /certified-nodes/ and /certified-nodes/* (paginated children).
• Make the certified filter URL-driven so the redirect lands in a pre-filtered view. (Shipped in Phase 1.)
• Audit inbound links to /certified-nodes (marketing, blog, handbook). One functional inbound link: the Certified Nodes feature tile on /platform/features/, rendered from src/_data/features.json. Updated to point at /integrations/?certified=1. Blog/changelog/docs references are historical content covered by the 301.
• Delete src/certified-nodes.njk, src/_includes/certified/template.njk, and src/_data/certifiedNodes.js. The ff-integrations API already returns ffCertified: true on each certified node, so catalog.flowfuse.com is redundant.
• Remove the redundant overlay in integrations.js:48-51 that re-derives ffCertified from certifiedNodes(). Refactored to use the API's ffCertified flag directly — also dropped the dependency on catalog.flowfuse.com.

Acceptance: /certified-nodes/ returns a 301 to a pre-filtered /integrations/. No build artifacts remain. No internal links 404.

Test plan:

• Visit /certified-nodes/ in a browser. URL bar updates to /integrations/?certified=1, page loads with the filter applied (toggle pressed, count shows 17).
• Visit /certified-nodes/page2/ (or any sub-path). Redirects the same way — splat covers paginated URLs.
• /integrations/ still shows all 17 certified nodes in the catalog (API-derived ffCertified flag is the sole source).
• On /platform/features/, the "Certified Nodes" tile's "Explore our certified nodes" link points to /integrations/?certified=1 (not /certified-nodes/).