Add only what is a feed or links one; refuse the rest (#102)

Adding an address looked for the feed a web page links and, finding none, added the address
as it was: every check then failed, and the sidebar called it a feed that had moved. cnn.com
is one; its page links no feed. find_feed replaces feed_behind_page: the address is added if
it is a feed or an OPML list, the feed its page links if it is a web page that links one (and
that is a feed), and otherwise the add is refused with the reason, from the web page (400, the
dialog stays open) and from `ipx add`.

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

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Adding an address checks it first: a feed is added, a web page adds the feed it links, and anything else is refused with the reason, instead of being added and failing on every check.
 - A feed that was failing when its last subscriber left is forgotten, rather than kept with its error for good. One that worked, or has files on disk, is kept as before.
 - A feed that keeps failing is checked less and less often, waiting as long as it has been failing, up to once a day; it goes back to its schedule as soon as it works. Refreshing it still checks it at once.
 - A scan's trace shows the time a feed spends on its artwork and in the database after the fetch.