Per Codex — make /a/<id> feel like Upbeat Bytes has editorial judgment, not just
a summary wrapper. Trust-building, short, not an essay.
* article_summaries gains what_happened / why_matters / why_belongs (+ migration).
* summarize.explain_article: a separate, fallback-able LLM pass producing three
short notes (parsed from a labelled WHAT/MATTERS/BELONGS format). generate_summary
now stores them alongside the summary, and tops up older summaries on next view.
get_explanation returns them only when all three are present.
* API: share_page + /api/summary expose the explanation.
* share.py: renders the three-part section (accent rule) when complete; otherwise
the single "Why it's here" reason line is the calm fallback. The page polls and
swaps in both the summary and the section as they cache.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
