Real-image posters: scale and encode in the background

Done in Draw it took about 200 ms per new poster (BiLinear scale to the
box plus PNG encoding), so each step to another title stalled the UI.

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

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- **Moving through lists is quick again with real-image posters:** scaling and encoding a poster
+  for the terminal (about 200 ms since posters are sent at the size of their box) now happens in
+  the background; the poster appears when it is ready, and the selection never waits for it.
 - **Trakt lists show faster after a restart:** the Radarr/Sonarr lookups of their titles are kept
   on disk too (3 days, `lookups.json`), so a list takes one request to Trakt and a quick library
   check per title instead of a lookup (about a second each) per title. Whether a title is in the