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>

CLAUDE.md

@@ -182,7 +182,8 @@ Non-trivial logic leaves one runnable check behind. Pure functions (`merge_polic
   command, the healthcheck's `ipx status` included, so it must never write when nothing is
   missing: SeaORM's experimental schema sync dropped and remade an index on every open, the
   write lock that took made `ipx status` time out behind a busy daemon, and it was removed for
-  it. A new column on an existing table needs its own `ALTER`; nothing adds one for you.
+  it. A new column on an existing table needs its own `ALTER`; nothing adds one for you. Look for
+  the column first and alter only when it is missing, as `feeds.description` does.
 * **SQL written by hand in `db.rs` has to run on SQLite and Postgres both** (issue #18): `$1`
   parameters, bound only if used; `ON CONFLICT`, not `INSERT OR IGNORE`; yes/no columns tested
   as themselves (`NOT coalesce(s.read, false)`) and written as `true`/`false`, never compared to