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/models.py

@@ -43,6 +43,8 @@ class Message(models.Model):
     completion_tokens = models.PositiveIntegerField(default=0)
     latency_ms = models.PositiveIntegerField(default=0)
     from_cache = models.BooleanField(default=False)
+    # شکلِ پاسخ (rag.services.diagram)؛ فقط توصیفِ JSONِ اعتبارسنجی‌شده، نه SVG.
+    diagram = models.JSONField(null=True, blank=True)
 
     class Meta:
         ordering = ['created_at']