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/components/ui/MixArt.svelte

@@ -0,0 +1,39 @@
+<script lang="ts">
+	import Cover from './Cover.svelte';
+	import { gradientForHue, hueFor } from '$lib/theme/color';
+	import { mixItemKey, type MixItem } from '$lib/mixes';
+
+	interface Props {
+		items: MixItem[];
+		hue: number;
+		class?: string;
+	}
+
+	let { items, hue, class: klass = '' }: Props = $props();
+
+	const tiles = $derived(items.slice(0, 4));
+</script>
+
+{#if tiles.length >= 4}
+	<div class="grid grid-cols-2 grid-rows-2 overflow-hidden {klass}">
+		{#each tiles as item (mixItemKey(item))}
+			<Cover
+				trackId={item.trackId ?? item.videoId ?? ''}
+				src={item.thumbnailUrl}
+				hue={hueFor(mixItemKey(item))}
+				size="small"
+				class="h-full w-full"
+			/>
+		{/each}
+	</div>
+{:else if tiles.length > 0}
+	<Cover
+		trackId={tiles[0].trackId ?? tiles[0].videoId ?? ''}
+		src={tiles[0].thumbnailUrl}
+		hue={hueFor(mixItemKey(tiles[0]))}
+		size="medium"
+		class={klass}
+	/>
+{:else}
+	<div class={klass} style="background:{gradientForHue(hue)}"></div>
+{/if}