Show what the pipeline understood about each article

A muted one-line note now follows every article: the deep assessment's
topic group, format, depth and technicality, the specific topics it
extracted, and the reader interests the article matched best, e.g.

  Software engineering · analysis essay · in depth · highly technical ·
  Topics: copy-on-write, ZFS · Interests: Filesystems, Rust

It renders on the web article page footer, on the public and private
issue-page entries, in the EPUB article footer and in the EPUB "In This
Issue" index (both editions), always styled like the existing meta lines
so it stays secondary to the content.

`Pick` gains `top_interests` (serde default, no migration), filled from
the signals stage when the editor assembles the lineup; the facets were
already on the issue snapshot via `Pick.llm`, so earlier issues show the
facet and topic parts and only future issues carry interests. One shared
builder, `understanding_line` in epub::chapters, feeds every surface.

The dev seed gives four picks facets and interests so the line can be
eyeballed locally.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QVPagF6jfDv78CC5Jv2wp4

src/epub/templates/chapter.xhtml

@@ -26,6 +26,9 @@
       </div>
       <hr class="rule"/>
       <div class="article-footer">
+{% if let Some(line) = understanding %}
+        <p class="understood">{{ line }}</p>
+{% endif %}
 {% if let Some(links) = rating %}
         <p class="rating">Was this a good pick? &#160; <a href="{{ links.loved_url }}">[ Loved it ]</a> &#160; <a href="{{ links.good_url }}">[ Good ]</a> &#160; <a href="{{ links.not_for_me_url }}">[ Not for me ]</a> &#160;&#160;&#160; <a href="{{ read_online_url }}">Read online &#8599;</a></p>
 {% else %}