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)/mixes/[id]/+page.server.ts

@@ -0,0 +1,28 @@
+import type { PageServerLoad, Actions } from './$types';
+import { createApiClient, requireUser, unwrapOrError } from '$lib/server/api';
+import { addTrackAction, addYouTubeToPlaylistAction } from '$lib/server/playlistActions';
+import { PLAYLIST_PICKER_PAGE_SIZE } from '$lib/config';
+
+export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
+	const user = requireUser(locals, url);
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+
+	const [mixRes, playlistsRes] = await Promise.all([
+		api.GET('/mixes/{id}', { params: { path: { id: params.id } } }),
+		api.GET('/playlists/users/{userId}', {
+			params: {
+				path: { userId: user.sub },
+				query: { pageNumber: 1, pageSize: PLAYLIST_PICKER_PAGE_SIZE }
+			}
+		})
+	]);
+
+	const mix = unwrapOrError(mixRes, 'Mezcla no encontrada.', 404);
+
+	return { mix, playlists: playlistsRes.data?.items ?? [] };
+};
+
+export const actions: Actions = {
+	addTrack: addTrackAction,
+	addYouTubeToPlaylist: addYouTubeToPlaylistAction
+};