ChipGroup is orphaned since the explore rewrite dropped the filter chips.
Mix.GeneratedOn was written on every generation and never read, and it
duplicated CreatedAt, which IAuditable already sets at the same moment.
Mixes are their own concept, not playlists: they live in their own
Mixes/MixItems tables, cannot be edited and never show up among the
user's playlists. Each item is either a Musify track or a YouTube Music
reference, so YouTube songs stream directly instead of being downloaded.
A daily Hangfire job seeds the mixes from the listening history: one
discovery mix, one daily mix and one per top artist.
The per-user YouTube filler query was already in the working tree and
rides along here because it shares files with this change.
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