a90ff11f0f
Adds ?kind= filter (user|system|all, default user) to GET /api/playlists so system mixes don't leak into slice-1 SPA views. Lazy background build fires when kind=system|all and the caller's run row is stale (>24h). Propagates Kind/SystemVariant/SeedArtistID through PlaylistRow and playlistRowView wire types. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>