A feed with no artwork takes its site's icon (#70)

When a feed names no image and none is stored, the scan fetches the
channel's site link (RSS <link>, Atom rel=alternate) and uses the
apple-touch-icon or icon it names, falling back to /favicon.ico when that
answers with an image.

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

CHANGELOG.md

@@ -9,6 +9,7 @@ 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.
 - `/api/status` gives the number of feeds, items waiting to download and files downloaded, and
   the version, for a dashboard such as Homepage.