feat(history): Implement conversation history management with artifact restoration and Markdown rendering.

backend/src/ea_chatbot/api/routers/history.py

@@ -59,7 +59,8 @@ async def get_conversation_messages(
             "id": str(m.id),
             "role": m.role,
             "content": m.content,
-            "created_at": m.created_at
+            "created_at": m.created_at,
+            "plot_ids": [str(p.id) for p in m.plots]
         } for m in messages
     ]
 

backend/src/ea_chatbot/api/schemas.py

@@ -37,7 +37,7 @@ class MessageResponse(BaseModel):
     role: str
     content: str
     created_at: datetime
-    # Plots are fetched separately via artifact endpoints
+    plot_ids: List[str] = []
 
 class ConversationUpdate(BaseModel):
     name: Optional[str] = None

backend/src/ea_chatbot/app.py

@@ -328,7 +328,8 @@ def main():
             "code_output": None,
             "error": None,
             "plots": [],
-            "dfs": {}
+            "dfs": {},
+            "iterations": 0
         }
 
         # Placeholder for graph output