feat(db): add contextual_likes table (M3 session-context capture)

Table was referenced in migration 0005's comment but never created —
this slice fills the gap. Soft-delete via deleted_at column; hot-path
partial index on active rows; GIN index on session_vector for
M3 sub-plan #3's similarity queries.

internal/db/dbq/models.go

@@ -29,6 +29,16 @@ type Artist struct {
 	UpdatedAt pgtype.Timestamptz
 }
 
+type ContextualLike struct {
+	ID            pgtype.UUID
+	UserID        pgtype.UUID
+	TrackID       pgtype.UUID
+	LikedAt       pgtype.Timestamptz
+	DeletedAt     pgtype.Timestamptz
+	SessionVector []byte
+	SessionID     pgtype.UUID
+}
+
 type GeneralLike struct {
 	UserID  pgtype.UUID
 	TrackID pgtype.UUID