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>

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- In the Directory, a feed you do not subscribe to opens a page of its own: its cover, its category, how many people here subscribe, and its latest episodes or posts, with a button to subscribe. Before, only its subscribe button did anything.
+- In the Directory, a feed you do not subscribe to opens a page of its own: its cover, its category, how many people here subscribe, what the feed says it is, and its latest episodes or posts, with a button to subscribe. Before, only its subscribe button did anything.
 - Feeds that don't set a category, as most blogs don't, now get one automatically, so they show up under a category in the Directory. iPX picks the closest of Apple's podcast categories from the feed's title and latest headlines. This happens once per feed: the first time a new feed is read, or for a feed you already had, the next time you force a refresh. It needs `TYPESAFE_KEY` set. An admin can change the category in the feed's settings.
 - Artwork is kept on disk once shown and loads from iPX, not from each publisher's server: fast after the first time, and still there when the publisher's server is not. The admin page sets how much is kept (500 MB by default).
 - `ipx add --list --category News <url>` puts a feed in the Directory for anyone to subscribe to,