Web dashboard v2 step 1: public summaries, legacy issue fallback

Public issue pages and the Atom feed now carry each pick's summary and
'Why it's here' line (bodies, The Brief, World Briefing and comments stay
private). Issues stored before issues.issue_json existed rebuild their
colophon and Behind the paper from issues.report_json / the run row and
candidate_runs, and recover the World Briefing chapter from the EPUB on
disk. The dev seed mirrors that legacy shape.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NHyYupFdBiR4VfoUM7NjSM

src/web/templates/world.html

@@ -1,5 +1,5 @@
 {% extends "layout.html" %}{% block content %}<article class="reading world-briefing">
-<h1>World Briefing</h1><p class="dateline">{{ display_date }}</p><hr class="rule">
+<h1>World Briefing</h1>{% match display_date %}{% when Some with (date) %}<p class="dateline">{{ date }}</p>{% when None %}{% endmatch %}<hr class="rule">
 <div class="world-body">{{ body_html|safe }}</div>
 <p><a href="{{ issue_href }}">← Back to this issue</a></p>
 </article>{% endblock %}