draw a diagram in tutor answers: validated JSON from the model, rendered by the app; add the fourth companion tedi

The model ends a shape question with <diagram>{json}</diagram>; rag/services/diagram.py
splits it off before mathtext, validates it against a whitelist and stores it on
Message.diagram (ai_tutor.0006). It reaches the app in the SSE done event and in
conversation history. Prompt tutor-v6-diagrams; a hint next to the question (shape
words only) raised gpt-4.1-mini from 3/12 to 12/12 diagrams, 0/8 on arithmetic.

progress.0005 adds tedi (600 xp) to the companion catalog.

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

ai_tutor/migrations/0006_message_diagram.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.17 on 2026-09-28 12:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ai_tutor', '0005_message_chunk_count_message_completion_tokens_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='message',
+            name='diagram',
+            field=models.JSONField(blank=True, null=True),
+        ),
+    ]