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.
The .gitignore 'downloads/' pattern for the DOWNLOAD_ROOT bind mount also matched the
backend/app/downloads source package, so formats.py/storage.py/service.py never got
committed with M2. Anchor the ignore to '/downloads/' (repo root only) and add the package.
storage.sanitize() now handles arbitrarily messy titles (emoji, ZWJ, fullwidth, clickbait
punctuation): NFKC-normalize, drop emoji/symbol/control unicode categories, collapse
repeated punctuation, underscore-join words -> space-free paths. Accents (HU/DE) preserved,
no ASCII folding. Plex layout uses _-_ separators + Season_{year}.
