Lesson.figures: fixed diagrams per lesson (stored once, served with the lesson); gen_lesson_figures; 3 calculus-12 lessons

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

content/seed_sync.py

@@ -126,7 +126,7 @@ def _sync_lesson(node, path, row, ledger, counts):
             return
         lesson = Lesson.objects.create(
             node=node, title=row['title'], intro_text=row['intro_text'],
-            example=row['example'], steps=row['steps'],
+            example=row['example'], steps=row['steps'], figures=row.get('figures') or [],
         )
         counts['lessons'] += 1
         ledger.record(_key('lesson', *path), 'lesson')
@@ -154,6 +154,9 @@ def _sync_lesson(node, path, row, ledger, counts):
     if row.get('steps') and not lesson.steps:
         lesson.steps = row['steps']
         changed.append('steps')
+    if row.get('figures') and not lesson.figures:
+        lesson.figures = row['figures']
+        changed.append('figures')
     if changed:
         lesson.save(update_fields=changed)
         counts['updated'] += 1