I delete the dump file, then decide to backup from that's date - restore will fail?
No, a copy a the postgresql file is in the backup. The file at /home/yellowtent/platformdata/<appid>/postgresqldump is a temporary file. When we want to backup an app, we create a dump from postgresql and save it to that file. Then we upload that file to wherever the backup is located . After the backup is done, that file has no real use but we just keep it around because in some rare cases where you don't have backups it can help in recovery.
So, to summarize:
Safe to delete /home/yellowtent/appsdata/app-container-id/postgresqldump . But this will be created as part of the backup process, so it's going to be a pain. You have to delete this file everyday.
Your final backup location will have a copy of the above file. Under <timestamp>/appid/postgresqldump . This is your backup, don't delete this file!