vlc: move persisted data to ~/private, and add .cache/vlc, for artwork

hosts/common/programs/vlc.nix

@@ -10,8 +10,13 @@ let
 in
 {
   sane.programs.vlc = {
+    persist.private = [
       # vlc remembers play position in ~/.config/vlc/vlc-qt-interface.conf
-    persist.plaintext = [ ".config/vlc" ];
+      # filenames are stored in plaintext (unlike mpv, which i think hashes them)
+      ".config/vlc"
+      # vlc caches artwork. i'm not sure where it gets the artwork (internet? embedded metadata?)
+      ".cache/vlc"
+    ];
     fs.".config/vlc/vlcrc".symlink.text = ''
       [podcast]
       podcast-urls=${podcast-urls}