refactor: standardize runtime annotation types across multiple files

backend/app/models/tasks.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from datetime import datetime  # noqa: TCH003
+from datetime import datetime
 from uuid import UUID, uuid4
 
 from sqlmodel import Field
@@ -10,6 +10,8 @@ from sqlmodel import Field
 from app.core.time import utcnow
 from app.models.tenancy import TenantScoped
 
+RUNTIME_ANNOTATION_TYPES = (datetime,)
+
 
 class Task(TenantScoped, table=True):
     """Board-scoped task entity with ownership, status, and timing fields."""