feat(web/playlists): Discover refresh button (detail + home kebab)

Operator can refresh their Discover playlist without waiting for
the next daily cron tick. Two surfaces, both gated on
system_variant === 'discover':

- Detail page header: "Refresh" button next to the cover art / meta.
- Home Playlists row tile kebab: "Refresh Discover" menu item.

Both call POST /api/playlists/system/discover/refresh, show a
confirmation toast, and invalidate the playlist + playlists queries
so the new content lands without a manual page reload.

Extends Playlist.system_variant union with 'discover'. Adds
refreshDiscover() typed client. Tests cover client behaviour
(success + empty-library) plus conditional rendering on both
surfaces.

web/src/lib/api/playlists.ts

@@ -91,3 +91,20 @@ export function createPlaylistQuery(id: string) {
     staleTime: 30_000
   });
 }
+
+// System playlist refresh ----------------------------------------------------
+
+export type RefreshDiscoverResponse = {
+  playlist_id: string | null;
+  track_count: number;
+};
+
+// Re-runs the daily system playlist build for the calling user and
+// returns the resulting Discover playlist's id + track count.
+// Used by the detail-page Refresh button and the home tile kebab.
+export async function refreshDiscover(): Promise<RefreshDiscoverResponse> {
+  return (await apiFetch('/api/playlists/system/discover/refresh', {
+    method: 'POST',
+    body: JSON.stringify({})
+  })) as RefreshDiscoverResponse;
+}