docs(latest): document cover extraction fallback

backend/internal/latest/sites.go

@@ -215,6 +215,9 @@ func comixCoverURL(seriesURL, body string) string {
 	return publishedCoverURL(detail.Poster.Medium)
 }
 
+// coverFrom reports false for unknown sites, challenge bodies, and pages with
+// no usable cover. Metadata extraction keeps scanning after an empty match so
+// a later published cover is not hidden by an empty tag.
 func coverFrom(site, seriesURL, body string) (string, bool) {
 	var cover string
 	switch site {