Show what a feed says it is on its page in the Directory (#130)

A feed's page in the Directory (#128) had its cover, category and latest items, but not the
feed's own description, which Apple's show page leads with: iPX read every item's description
and threw the channel's away, and the feeds table had nowhere to keep it.

It is parsed now, RSS's <description>, or iTunes' summary when that is empty, or Atom's
subtitle, kept in feeds.description (kept when a later read has none, as title and image are),
and sent, sanitized, with the items from /api/directory/{id}, which is now an object, not a
list. The page shows it as plain text under the header, three lines of it, with More when there
is more. A description that only repeats the title is left out.

feeds.description is the first column added to a table that already exists. create_missing
looks for it with a SELECT and runs the ALTER only when it is missing, since it runs on every
open, the healthcheck's included, and an ALTER's lock is what made that time out before. The
same once-only step forgets every feed's ETag and Last-Modified: a feed is read whole only when
it has changed, so its description would otherwise wait for its next item.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/users.md

@@ -75,8 +75,8 @@ with the most subscribers on this server, you included, ranked, then the categor
 showing a few of its shows' covers. A category has a page of its own: its most subscribed feeds,
 then all of them as a grid of cover art, with its subcategories along the top (Tech News under
 News, Video Games under Leisure) and a choice of order. See all lists every feed, and the search
-box finds one by name. A feed you do not subscribe to opens a page of its own, with its latest
-episodes or posts, so you can see what it is first. Every page can be narrowed to Podcasts (anything with audio or video) or
+box finds one by name. A feed you do not subscribe to opens a page of its own, with what it says it
+is and its latest episodes or posts, so you can see what it is first. Every page can be narrowed to Podcasts (anything with audio or video) or
 Blogs (the rest). A show's category is the one it gives itself in iTunes, or for one that names
 none, one an admin or iPX picked for it. Your own feeds are marked Subscribed.
 It shows a title, artwork and a count, never a URL or who reads it. An OPML subscription is listed