feat(blog): add tag archive pages, related posts, and per-tag RSS feeds

[Escelit]

Closes #131

Summary

Implements #131 — triples internal linking surface on the blog via tag archives, related posts, and per-tag RSS feeds.

• Tag archive route — /blog/tag/:slug lists every post carrying that tag, most recent first, and links its per-tag feed.
• Related posts block on single-post pages picks up to 3 posts by highest tag overlap, excluding the current post.
• Per-tag RSS — scripts/gen-rss.mjs now emits /feed/tag/:slug.xml for every tag alongside the global /feed.xml. Each blog post references a per-tag <link rel="alternate"> for each of its tags; the archive references its own.
• Sitemap — both scripts/sitemap.ts (build) and scripts/gen-sitemap.mjs now include every /blog/tag/:slug URL.
• Added two MDX posts so the (MDX-only) blog corpus has overlapping tags for the archive/related features to be real.

Test plan

• pnpm format:check, pnpm lint:i18n, pnpm test:a11y, pnpm build all pass locally (CI build job steps).
• src/__tests__/blog.test.tsx and src/__tests__/rss.test.ts cover tag resolution, related-posts overlap/exclusion, tag archive rendering, and per-tag feed generation.
• Generated public/feed/tag/*.xml and sitemap.xml verified to include all 6 tag archives.

[vercel]

@Escelit is attempting to deploy a commit to the truthixify's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@Escelit Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[truthixify]

Thanks @Escelit. Conflicts on src/pages/Blog.tsx against #136, which landed reading-time and the TOC.

Suggest holding this one until #138 merges, then rebasing. I left the reasoning on #138: your three blog PRs all touch this file, so landing them in a fixed order means one rebase each instead of repeated ones.

[Escelit]

Thanks @Escelit. Conflicts on src/pages/Blog.tsx against #136, which landed reading-time and the TOC.

Suggest holding this one until #138 merges, then rebasing. I left the reasoning on #138: your three blog PRs all touch this file, so landing them in a fixed order means one rebase each instead of repeated ones.

Successfully rebased on #136

[Escelit]

The rebase is suceesfully done. Please review

[truthixify]

Heads up: this went conflicted a few minutes ago because #146 (your author pages), which was first in the order we agreed landed on develop. It was mergeable right before that, so nothing is wrong with your work.

git fetch origin
git rebase origin/develop

and it should be good to go.

[truthixify]

Merged, second of your three @Escelit. Per-tag feeds plus the sitemap picking up the tag archives means the internal linking actually compounds. #140 is last, and it will need one more rebase now that this has landed.