A site icon that is missing is not used (#79)

site_icon took the icon a site's page names in its <link> tags without
asking for it, so a dead one was stored and /favicon.ico never tried.
antirez.com names /images/favicon.png, which is a 404, while its
/favicon.ico is there; the feed showed no artwork, and since the lookup
happens once, never would. The named icon now has to answer with an
image, as /favicon.ico already did.

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

CHANGELOG.md

@@ -32,6 +32,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A feed whose website names an icon that is missing shows the site's `/favicon.ico` instead of
+  no artwork.
 - The read button shows whether an item is read, as the pin beside it shows whether it is
   pinned: a tick when read, an envelope when not. It used to show the opposite.
 - In Directory and Popular, Subscribe is a plus again, not the tick that marks a feed you have.