feat(history): Implement get_messages_by_window in HistoryManager

backend/src/ea_chatbot/history/manager.py

@@ -185,4 +185,28 @@ class HistoryManager:
             # Pre-load plots for each message
             for m in messages:
                 _ = m.plots
-            return messages
+            return messages
+
+    def get_messages_by_window(self, conversation_id: str, window_size: int = 10) -> List[Message]:
+        """
+        Get the last N messages for a conversation, ordered by creation time (ascending).
+        """
+        with self.get_session() as session:
+            # 1. Get the last N messages in descending order
+            stmt = (
+                select(Message)
+                .where(Message.conversation_id == conversation_id)
+                .order_by(Message.created_at.desc())
+                .limit(window_size)
+            )
+            result = session.execute(stmt)
+            messages = list(result.scalars().all())
+            
+            # 2. Reverse to get ascending chronological order
+            messages.reverse()
+            
+            # Pre-load plots if needed (though graph usually only needs text)
+            for m in messages:
+                _ = m.plots
+                
+            return messages