refactor: standardize runtime annotation types across multiple files

backend/app/schemas/metrics.py

@@ -2,11 +2,13 @@
 
 from __future__ import annotations
 
-from datetime import datetime  # noqa: TCH003
+from datetime import datetime
 from typing import Literal
 
 from sqlmodel import SQLModel
 
+RUNTIME_ANNOTATION_TYPES = (datetime,)
+
 
 class DashboardSeriesPoint(SQLModel):
     """Single numeric time-series point."""