Trakt lists: keep the Radarr/Sonarr lookups of their titles on disk

Library membership is asked every time via movie?tmdbId= / series?tvdbId=;
only the lookups of titles not in the library are stored (3 days).

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

README.md

@@ -14,7 +14,8 @@ A terminal UI to search for movies and series with [Radarr](https://radarr.video
   ticked, the others can be added right there (Enter). Needs a free Trakt client ID (see Configure).
 - **Library**: movies and series with their status; the dot before a title turns blue while it
   is downloading, and the details say how far.
-- **Cache**: posters (30 days, at most 200 MB) and casts (7 days) are kept between runs in
+- **Cache**: posters (30 days, at most 200 MB), casts (7 days) and the Radarr/Sonarr lookups of
+  Trakt list titles (3 days) are kept between runs in
   `~/.cache/arr-tui` (`$XDG_CACHE_HOME`; `~/Library/Caches/arr-tui` on macOS), so they show at
   once after a restart. Deleting the folder is always safe.
 - **Season posters**: in a series' seasons and episodes, the poster follows the selected season.