fix: improve maintenance and synchronize live conversations

backend/app/repositories/chat.py

@@ -21,7 +21,6 @@ from app.models.chat_history import ChatHistory
 from app.models.customer import Customer
 from uuid import UUID
 from sqlalchemy import func, or_, select, text, and_
-from sqlalchemy.sql import case
 from app.models.agent import Agent
 from app.models.session_to_agent import SessionToAgent
 from app.repositories.channels import ChannelConversationRepository