Show artwork up to 20 MB, and remember and log what failed (#169)

Five Glass Cannon Network feeds name Patreon covers of about 7 MB, over the
5 MB limit on /api/art. Each page load fetched them all again, refused them,
and logged a 502 per cover, about 84 a day, with nothing in the log to say
which image or why: the reason went only to the browser. The limit is now
20 MB, a failure is answered from memory for an hour, and the fetch failure
is logged with its address and reason.

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

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Cover art up to 20 MB is shown, instead of nothing for anything over 5 MB, as for five of the Glass Cannon Network's feeds.
+- Artwork that could not be fetched is not fetched again for an hour, instead of on every page load, and the server's log says which image it was and why.
+
 ## [0.10.3] - 2026-10-09
 
 ### Changed