feat(db/m7-352): sqlc queries for system playlist runs + seed selection

Adds internal/db/queries/system_playlists.sql with 12 named queries
covering active-user enumeration, per-user run tracking (claim/finish/fail),
seed-artist/track/cover selection, and playlist CRUD by kind.
Runs sqlc generate to emit dbq/system_playlists.sql.go; also updates
playlists.sql.go and models.go to reflect the new kind/system_variant/
seed_artist_id columns added in migration 0015.

Note: plan specified a.cover_path for PickTopAlbumCoverForArtistByUser
but albums uses cover_art_path — corrected in the query file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/dbq/models.go

@@ -336,16 +336,19 @@ type PlaySession struct {
 }
 
 type Playlist struct {
-	ID          pgtype.UUID
-	UserID      pgtype.UUID
-	Name        string
-	Description string
-	IsPublic    bool
-	CoverPath   *string
-	TrackCount  int32
-	DurationSec int32
-	CreatedAt   pgtype.Timestamptz
-	UpdatedAt   pgtype.Timestamptz
+	ID            pgtype.UUID
+	UserID        pgtype.UUID
+	Name          string
+	Description   string
+	IsPublic      bool
+	CoverPath     *string
+	TrackCount    int32
+	DurationSec   int32
+	CreatedAt     pgtype.Timestamptz
+	UpdatedAt     pgtype.Timestamptz
+	Kind          string
+	SystemVariant *string
+	SeedArtistID  pgtype.UUID
 }
 
 type PlaylistTrack struct {
@@ -388,6 +391,14 @@ type SkipEvent struct {
 	PositionMs int32
 }
 
+type SystemPlaylistRun struct {
+	UserID      pgtype.UUID
+	LastRunAt   pgtype.Timestamptz
+	LastRunDate pgtype.Date
+	InFlight    bool
+	LastError   *string
+}
+
 type Track struct {
 	ID          pgtype.UUID
 	Title       string