Add sensor freshness checks

custom_components/heating_mpc/data_quality.py

@@ -0,0 +1,18 @@
+"""Pure timestamp validation for reported sensor snapshots."""
+from __future__ import annotations
+
+from datetime import datetime
+from math import isfinite
+
+
+def freshness(reported: datetime | None, now: datetime, max_age_seconds: float) -> dict:
+    """Use report time, not value-change time; reject unknown/future clocks."""
+    if not isfinite(max_age_seconds) or max_age_seconds <= 0:
+        raise ValueError("Freshness limit must be positive and finite")
+    if not isinstance(reported, datetime) or reported.tzinfo is None:
+        return {"status": "invalid_timestamp", "age_seconds": None}
+    age = (now - reported).total_seconds()
+    if age < 0:
+        return {"status": "future_timestamp", "age_seconds": round(age, 1)}
+    return {"status": "stale" if age > max_age_seconds else "valid",
+            "age_seconds": round(age, 1)}