fix(scheduler): give the download_gc job a proper name + tooltip

The download-center GC job showed its raw id 'download_gc' with a raw-key tooltip, unlike every
other job. Add its label ('Download cleanup') + description to scheduler i18n (en/hu/de), and make
the job tooltip fall back to empty for any job lacking a description (so a future/edge job never
shows a raw 'scheduler.jobDesc.*' key — also fixes demo_reset).

frontend/src/components/Scheduler.tsx

@@ -142,7 +142,7 @@ function JobRow({
       <StatusDot k={statusKey(job)} />
       <div className="min-w-0 flex-1">
         <div className="text-sm font-medium flex items-center gap-1.5 flex-wrap">
-          <Tooltip hint={t(`scheduler.jobDesc.${job.id}`)}>
+          <Tooltip hint={t(`scheduler.jobDesc.${job.id}`, "")}>
             <span className="underline decoration-dotted decoration-muted/40 underline-offset-4 cursor-help">
               {t(`scheduler.jobs.${job.id}`, job.id)}
             </span>