Add JSON API, RSS feed, and related tools

[gronnerup]

Summary

Three catalog-platform additions that make the data reusable and improve discovery: a public JSON API, an RSS feed of recent additions, and related-tools on each detail page.

Changes

• JSON API (/api/tools.json): read-only export of every entry (frontmatter + detail-page href), sorted by title, with count and CC-BY-4.0 license noted. public/_headers adds Access-Control-Allow-Origin: * so browser apps can fetch it cross-origin.
• RSS feed (/rss.xml, via @astrojs/rss): 50 most recently added tools, with autodiscovery <link> in the head so feed readers find it.
• Related tools (tool detail pages): up to 3 tools scored by shared category (+2) and overlapping tags (+1 each), rendered with the existing ToolCard.
• Footer now links RSS and API for discoverability.

Verification

• npx astro check -> 0 errors/warnings/hints.
• npm run build passes; verified dist/api/tools.json (56 tools), dist/rss.xml, dist/_headers, and a rendered "Related tools" section.

Notes

• Static-friendly: both endpoints prerender to files; CORS is delivered via Cloudflare Pages _headers (static hosts don't apply endpoint Response headers).
• BaseLayout edits are in different regions than the open site-polish PR (Add dark-mode toggle and site polish #27) to avoid a merge conflict.