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/routes/(app)/playlists/[id]/+page.server.ts

@@ -8,7 +8,8 @@ import {
 	unwrapOrFail
 } from '$lib/server/api';
 import { uploadPresignedImage } from '$lib/server/upload';
-import { fetchYoutubeFiller, shuffle } from '$lib/server/youtube';
+import { fetchYoutubeFiller } from '$lib/server/youtube';
+import { shuffle } from '$lib/collections';
 import { addTrackAction, addYouTubeToPlaylistAction } from '$lib/server/playlistActions';
 import type { YouTubeSong } from '$lib/types';