feat: recommend server-generated mixes from Musify and YouTube Music
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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