Uploads library on the USB drive with a nightly stable-disk copy; Topic channels resolve by name

- UPLOAD_DIR moves to the USB drive; reads fall back to UPLOAD_BACKUP_DIR on the
  ytplayer-data volume, and while the drive is offline new uploads land there
- scripts/ops/uploads-backup.sh (homelab cron, 03:30) copies both ways, never
  deletes; the server removes a deleted upload from both places
- /api/channel resolves a bare channel name ('Artist - Topic') to its id via a
  search, and the client falls back to the channel name for old saved entries

frontend/app.js

@@ -7519,7 +7519,9 @@ async function openChannel(channelKey, displayName) {
 
 // The identifier we hand the backend to look a channel up (URL preferred).
 function channelKeyOf(v) {
-  return (v && (v.channelUrl || v.channelId)) || '';
+  // Older saved entries may only have the channel's name; the server resolves
+  // a name (e.g. "Petra - Topic") to its channel id.
+  return (v && (v.channelUrl || v.channelId || v.channel)) || '';
 }
 
 // ============================================================================