Adding a site's address subscribes to the feed it links (#71, #72)

Both add paths, the CLI's and the web's, look behind the URL first: a web
page that names its feed with <link rel="alternate"> is swapped for that
feed, before the duplicate check so it finds a feed someone already has.
Before, the page itself was added and every scan failed on it.

alternate_feed_link found tags in a to_lowercase() copy and sliced the
original at those offsets; Unicode lowercasing changes some characters'
length, so a page with one before its <link> tags lost the href or
panicked off a char boundary. ASCII lowercasing keeps offsets aligned.

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
 ### Added
 
 - A feed that has no artwork of its own shows its website's icon instead.
+- Adding a website's address subscribes to the feed that site links, instead of failing on every scan.
 - `/api/status` gives the number of feeds, items waiting to download and files downloaded, and
   the version, for a dashboard such as Homepage.
 
@@ -29,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A web page with some non-ASCII characters no longer hides the feed it links, or crashes looking for it.
 - Pulling the item list down to check for new items shows a spinner for a couple of seconds, and
   a second pull meanwhile does nothing, instead of no sign at all that the check started.
 - Settings no longer lists the server's download folder, which only an admin can change, on