expose per-node lessonIds in GET /api/home

The skill-map response gave units -> nodes with id/title/status but no way
to map a node to its lessons, and no other endpoint fills that gap
(/api/progress/nodes doesn't either). Any client rendering the map needs
this to know which lesson to open when a node is tapped.

The ids are already computed in the cached _grade_tree; this just surfaces
them. Additive and backward-compatible; content tests pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/serializers.py

@@ -22,6 +22,9 @@ class NodeStatusSerializer(serializers.Serializer):
     id = serializers.CharField()
     title = serializers.CharField()
     status = serializers.ChoiceField(choices=['locked', 'unlocked', 'completed'])
+    # شناسه‌ی درس‌های همین گره تا کلاینت بدونه با کلیک روی گره کدوم درس رو
+    # باز کنه (endpointهای lessons/{id}/... مستقیم با همین id کار می‌کنن).
+    lessonIds = serializers.ListField(child=serializers.IntegerField())
 
 
 class UnitWithNodesSerializer(serializers.Serializer):