Harden the citation cache; reject paper_author outside paper types

[bits-bytes-nn]

Follow-ups from an adversarial review of main at ea6ee73. Both the code/tests/docs review and the UI/UX review found 0 MAJOR issues; this PR applies their cheap MINOR fixes.

• js/citations.js:
  • A failed refresh (429, 404, network) now re-stamps the cached count, so the visitor's browser waits a day before asking again instead of retrying on every view.
  • A cache timestamp in the future no longer counts as fresh.
  • An entry without a paperId is not linked.
  • The expected failure is logged with console.warn.
• script/lint-posts.rb: new rule. paper_author is rejected outside the paper types, as paper_doi already was; the layout prints "Paper by …" for any post that has it.
  • Passes: a Paper Reviews post with paper_author.
  • Fails: a Tech Guides post with paper_author: "Mistral AI" (paper_author is for Paper Reviews and Paper Summaries posts).
  • No current post is affected.
• Docs: docs/tech-doc.md lists all three scripts, and the skip-link comment counts the seventh tab.

_data/post_conventions.yml is unchanged.

Verification:

• tests: 174 runs, 0 failures
• lint: 35 posts pass
• strict build, htmlproofer and validate-site.sh all pass
• Playwright with the API mocked: a 200 response, a reload within 24h with no request, 429 with and without a cache, a blocked request, and the 5s timeout