Keep posters and casts between runs

Posters (the downloaded bytes, per size, 30 days, at most 200 MB) and
casts (people.json, 7 days, at most 2000) are stored in the user's cache
directory (os.UserCacheDir()/arr-tui) and used on the next start before
asking the servers. Expired posters are pruned at startup, casts are
written shortly after new ones arrive and on exit. Tests turn the cache
off.

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

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- **Posters and casts are kept between runs** (in `~/.cache/arr-tui`; posters 30 days and at
+  most 200 MB, casts 7 days), so they show at once after a restart and are not downloaded again.
 - **Downloading titles in the Library:** the dot before a movie or series turns blue while it is in
   the Radarr/Sonarr download queue, and its details say how far ("Downloading 45% · 0:06:12 left").
 - **After a grab** the release list closes and you are back where the search started.