Fetch artwork offered only over http for the https page (#90)

Through ipodderx.sdf1.net the page is https, the browser upgrades an http:// image to https,
and a host with no https, such as The Secret Cabal's CDN, answers nothing, so no artwork. On an
https page, the page now asks /api/art for those, and ipx fetches them. It only fetches an
address some feed or entry names as its artwork, and only an image, up to 5 MB, so the route
cannot be pointed at anything else on the network.

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

CHANGELOG.md

@@ -44,6 +44,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Artwork a feed offers only over plain http shows on the https site too.
 - A failing feed is easy to spot in any theme: a mark on its artwork, what is wrong in place of
   its counts, and its artwork greyed out once it has failed for a day.
 - A feed whose own artwork is missing shows its website's icon instead of nothing.