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.

web-player/src/lib/config.ts

@@ -9,3 +9,4 @@ export const LIBRARY_PICKER_PAGE_SIZE = 50;
 export const EXPLORE_ALBUMS_PAGE_SIZE = 18;
 export const HOME_LATEST_PAGE_SIZE = 12;
 export const YOUTUBE_FILLER_LIMIT = 27;
+export const PLAYLIST_PICKER_PAGE_SIZE = 50;