refactor: reorganize backend module boundaries

app/agent/callback/__init__.py

@@ -6,7 +6,7 @@ from fastapi.concurrency import run_in_threadpool
 
 from app.agent.policy import sanitize_for_host
 from app.chain import ChainBase
-from app.platform.log import logger
+from app.runtime.log import logger
 from app.schemas import Notification
 from app.schemas.message import (
     MessageResponse,