The engine worker stores the encoded filename under the key 'filename'
(see worker.py dispatch). _record_to_status was reading 'original_filename',
which always returns "" — so every DownloadStatus emitted by
get_all_downloads/get_download_status had an empty filename string.
The download monitor builds lookup keys as
_make_context_key(download.username, download.filename). With filename=""
the key was always "amazon::" which never matched the task's
"amazon::B0B1234||Artist - Title" key. Monitor never detected Amazon
download completions, so tasks sat stuck at Downloading 0% forever even
though the files had actually downloaded.
Also fixes tests that had the same wrong key.
ebda0b8613