A feed's own artwork has to be there before it is used (#89)

A feed's itunes:image or <image><url> was stored without being asked
for, so a dead one stood in the way of the site's icon. Ken and Robin
Talk About Stuff names http://kenandrobin.wpengine.com/.../kartas_podcast.png,
a 404, while its site's apple-touch-icon works. The feed's artwork now has
to answer as an image, as the site icon already did, when the feed is
read in full; otherwise the site's icon is looked for.

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

CHANGELOG.md

@@ -42,6 +42,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A feed whose own artwork is missing shows its website's icon instead of nothing.
 - The log has no terminal colour codes when it is not going to a terminal, as in `docker logs`.
 - A feed whose website names an icon that is missing shows the site's `/favicon.ico` instead of
   no artwork.