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>

src/main.rs

@@ -1084,6 +1084,7 @@ async fn scan_one(
         last_modified.as_deref(),
         parsed.ttl_mins,
         parsed.image.as_deref(),
+        parsed.category.as_deref(),
     )?;
 
     let policy = policy_for(ctx, id, feed_cfg)?;