feat: expose message source field in messages API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prompt/api.py

@@ -43,6 +43,7 @@ def list_messages(request, conversation_id: UUID):
         {
             "id": m.id,
             "role": m.role,
+            "source": m.source,
             "content": m.content,
             "code_html": m.code_html,
             "code_css": m.code_css,

prompt/schemas.py

@@ -6,6 +6,7 @@ from ninja import Schema
 class MessageOut(Schema):
     id: int
     role: str
+    source: str
     content: str
     code_html: Optional[str] = None
     code_css: Optional[str] = None