Run discover-overlay-wait as a user unit and rework the vidthumb cache

vidthumb names thumbnails by a hash of path, size and mtime instead of a jq index that broke on quotes in filenames; systemd-tmpfiles removes ones unused for 30 days. discover-overlay-wait now runs from a systemd user unit started by xmonad and logs to the journal.

.local/bin/discover-overlay-wait

@@ -1,4 +1,7 @@
 #!/bin/sh
+# Runs discover-overlay while Discord's RPC port is up and stops it when Discord exits.
+# Run by discover-overlay-wait.service (started from xmonad); output goes to the journal.
+
 port_up() {
     ss -ltn | grep -qE '127\.0\.0\.1:64[67][0-9]'
 }
@@ -8,7 +11,7 @@ while true; do
     sleep 5
     port_up || continue   # port vanished again, Discord was still restarting
 
-    discover-overlay >> /tmp/discover-overlay.log 2>&1 &
+    discover-overlay &
     pid=$!
 
     while port_up && kill -0 "$pid" 2>/dev/null; do sleep 3; done