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>

src/main.rs

@@ -1258,6 +1258,14 @@ async fn scan_one(
     // is stored as "", which the page draws as no art, and which stops the refetch above.
     // ponytail: a feed without validators is read in full every scan and asks its site each
     // time too; keep a checked-at time per feed if that shows up in anyone's logs.
+    // A feed's own artwork has to be there too: Ken and Robin's names a 404, and stored unasked
+    // it stood in the way of the site's icon, which works (#89).
+    if let Some(art) = &parsed.image
+        && !feed::is_image(&ctx.client, art).await
+    {
+        tracing::info!(feed = id, art = %art, "the feed's artwork is not an image; trying its site's icon");
+        parsed.image = None;
+    }
     if parsed.image.is_none() {
         parsed.image = Some(match &parsed.site {
             Some(site) => feed::site_icon(&ctx.client, site).await.unwrap_or_default(),