Two UAT findings:
1. The device download filename (Content-Disposition) kept emoji/symbols from the video title.
Add storage.display_filename (drops emoji/symbol/control unicode, keeps spaces + accents)
and use it for the download name — readable and clean ("…alapján!.mp4", no emoji).
2. Deleting/canceling a download removed the job but the shared MediaAsset (and its file) lingered
as cache, so 'Ready files' stayed inflated and disk wasn't freed. Rework: _release_asset drops
the hold and, once no job holds the asset, deletes the file + row immediately (the cache only
needs to span overlapping holders). Also fixes cancel never decrementing (it flipped status to
'canceled' before releasing, tripping the holding-state guard).
Verified: filename emoji-stripped; enqueue→delete removes the asset row + file from disk.
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