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/mixes.ts

@@ -0,0 +1,37 @@
+import type { components } from '$lib/api/schema';
+import type { TrackTarget } from '$lib/tracks';
+
+export type Mix = components['schemas']['MixApplicationResponse'];
+export type MixItem = components['schemas']['MixItemApplicationResponse'];
+
+export function mixItemKey(item: MixItem): string {
+	return item.source === 'YouTube' ? `yt:${item.videoId}` : `mf:${item.trackId}`;
+}
+
+export function mixItemTarget(item: MixItem): TrackTarget {
+	if (item.source === 'YouTube') {
+		return {
+			kind: 'youtube',
+			song: {
+				videoId: item.videoId ?? '',
+				title: item.title,
+				artist: item.artist ?? '',
+				album: '',
+				durationSeconds: Number(item.durationSeconds),
+				thumbnailUrl: item.thumbnailUrl ?? '',
+				isExplicit: item.isExplicit
+			}
+		};
+	}
+
+	return {
+		kind: 'local',
+		track: {
+			id: item.trackId ?? '',
+			title: item.title,
+			artist: item.artist,
+			source: 'Local',
+			isExplicit: item.isExplicit
+		}
+	};
+}