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/+page.server.ts

@@ -1,6 +1,7 @@
 import type { PageServerLoad, Actions } from './$types';
 import { createApiClient, playlistCoverTrackIds, requireUser } from '$lib/server/api';
-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 { HOME_LATEST_PAGE_SIZE, YOUTUBE_FILLER_LIMIT } from '$lib/config';
 import { pickGreeting } from '$lib/server/greeting';
@@ -15,13 +16,15 @@ export const load: PageServerLoad = async ({ locals, url, fetch }) => {
 	const playlistsPromise = api.GET('/playlists/users/{userId}', {
 		params: { path: { userId: user.sub }, query: { pageNumber: 1, pageSize: 12 } }
 	});
+	const mixesPromise = api.GET('/mixes');
 	const recentlyPlayedPromise = api.GET('/users/{id}/listening-history', {
 		params: { path: { id: user.sub } }
 	});
 
-	const [latest, playlists, recentlyPlayed] = await Promise.all([
+	const [latest, playlists, mixes, recentlyPlayed] = await Promise.all([
 		latestPromise,
 		playlistsPromise,
+		mixesPromise,
 		recentlyPlayedPromise
 	]);
 	const playlistItems = playlists.data?.items ?? [];
@@ -34,8 +37,15 @@ export const load: PageServerLoad = async ({ locals, url, fetch }) => {
 		greeting: pickGreeting(new Date().getHours()),
 		latest: shuffle((latest.data?.items ?? []).map((track) => ({ kind: 'local' as const, track }))),
 		latestHasNext: Boolean(latest.data?.hasNextPage),
-		youtubeFiller: fetchYoutubeFiller(api, locals.accessToken, YOUTUBE_FILLER_LIMIT),
+		youtubeFiller: fetchYoutubeFiller(
+			api,
+			locals.accessToken,
+			YOUTUBE_FILLER_LIMIT,
+			undefined,
+			user.sub
+		),
 		playlists: playlistItems,
+		mixes: mixes.data ?? [],
 		trackIds,
 		recentlyPlayed: recentlyPlayed.data ?? []
 	};