Say on a feed's Directory page why it is failing, and who subscribes (#142)

daily-quests, one subscriber and no items, showed "Nothing read from it yet. A feed nobody here
subscribes to is checked once a day." on its page in the Directory: the second sentence was
written whatever the count, and the feed had been failing since #119 said so, which the page
never showed, /api/directory/{id} sending no error.

It sends `failing` now, the reason a feed's own page gives (explain_failure), or that the last
check failed, never the error itself, which can name the feed's address, and the Directory names
none. The page shows it under the header in the error colour, and says a feed is checked once a
day only when nobody subscribes and nothing is wrong.

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

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - A publisher's correction to an item already in iPX, a retitled episode, mended text, new artwork or a fixed length, arrives on the next read. Since 0.9.1 an item kept what it first said: Ain't It Cool News's items each showed the text of the one before, long after the feed was fixed.
+- A feed's page in the Directory says when the feed is failing, and why, and no longer says nobody here subscribes to a feed someone does.
 
 ## [0.10.1] - 2026-10-05