Directory: chips by kind and category over a grid of cover art

Feeds take their channel's first <itunes:category> into a new feeds.category
column; the migration drops ETag and Last-Modified once so every feed re-reads
on its normal schedule and picks one up. /api/popular and /api/directory carry
category and podcast (any audio or video enclosure). Directory becomes a grid of
cover-art tiles under a chip rail: All, Podcasts, Blogs, and a podcast's
categories once Podcasts is picked. Popular and Add a feed keep their rows.

Closes #4, closes #5, closes #6, closes #7.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/data/rss2.xml

@@ -6,6 +6,8 @@
     <description>A synthetic feed used by the parser tests.</description>
     <ttl>45</ttl>
     <itunes:explicit>no</itunes:explicit>
+    <itunes:category text="Technology"><itunes:category text="Podcasting"/></itunes:category>
+    <itunes:category text="News"/>
 
     <item>
       <title>Episode One</title>