feat(server/m7-352): kind filter on GET /api/playlists + lazy fallback

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>

internal/playlists/service.go

@@ -65,6 +65,9 @@ type PlaylistRow struct {
 	Name          string
 	Description   string
 	IsPublic      bool
+	Kind          string      // 'user' | 'system'
+	SystemVariant *string     // 'for_you' | 'songs_like_artist' | nil
+	SeedArtistID  pgtype.UUID // Valid=true only for 'songs_like_artist'
 	CoverPath     *string
 	TrackCount    int32
 	DurationSec   int32
@@ -498,6 +501,9 @@ func playlistFromGetRow(r dbq.GetPlaylistRow) *PlaylistRow {
 		Name:          r.Name,
 		Description:   r.Description,
 		IsPublic:      r.IsPublic,
+		Kind:          r.Kind,
+		SystemVariant: r.SystemVariant,
+		SeedArtistID:  r.SeedArtistID,
 		CoverPath:     r.CoverPath,
 		TrackCount:    r.TrackCount,
 		DurationSec:   r.DurationSec,
@@ -514,6 +520,9 @@ func playlistFromListRow(r dbq.ListPlaylistsForUserRow) *PlaylistRow {
 		Name:          r.Name,
 		Description:   r.Description,
 		IsPublic:      r.IsPublic,
+		Kind:          r.Kind,
+		SystemVariant: r.SystemVariant,
+		SeedArtistID:  r.SeedArtistID,
 		CoverPath:     r.CoverPath,
 		TrackCount:    r.TrackCount,
 		DurationSec:   r.DurationSec,