refactor: standardize runtime annotation types across multiple files

backend/app/models/agents.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from datetime import datetime  # noqa: TCH003
+from datetime import datetime
 from typing import Any
 from uuid import UUID, uuid4
 
@@ -12,6 +12,8 @@ from sqlmodel import Field
 from app.core.time import utcnow
 from app.models.base import QueryModel
 
+RUNTIME_ANNOTATION_TYPES = (datetime,)
+
 
 class Agent(QueryModel, table=True):
     """Agent configuration and lifecycle state persisted in the database."""