feat: implement multi-user credits redeem and storage isolation

downloader.go

@@ -71,6 +71,11 @@ func (a *App) finishExisting(t *Task, size int64) {
 	}
 	t.TotalBytes, t.DownloadedBytes, t.Progress, t.Status = size, size, 100, "done"
 	a.db.Save(t)
+	// Existing local media is reused and therefore does not consume a credit.
+	if err := a.refundCredit(t); err != nil {
+		a.fail(t, err)
+		return
+	}
 	a.upload(t)
 }