Replace stale runway history on raw backfill

tests/test_latest_observation_overlay.py

@@ -152,10 +152,16 @@ def test_overlay_builds_amsc_runway_history_from_raw_store():
     result = overlay_latest_amsc_observation(
         FakeDB(),
         "chengdu",
-        {"name": "chengdu", "temp_symbol": "°C", "runway_plate_history": {}},
+        {
+            "name": "chengdu",
+            "temp_symbol": "°C",
+            "runway_plate_history": {
+                "02L/20R": [{"time": "2026-06-12T10:44:00+00:00", "temp": 31.6}],
+            },
+        },
     )
 
-    assert result["runway_plate_history"]["02L/20R"][-2:] == [
+    assert result["runway_plate_history"]["02L/20R"] == [
         {"time": "2026-06-15T11:04:00+00:00", "temp": 25.6},
         {"time": "2026-06-15T11:08:00+00:00", "temp": 25.4},
     ]

web/services/latest_observation_overlay.py

@@ -368,7 +368,7 @@ def _append_amsc_runway_history_from_raw_store(
     except Exception as exc:
         logger.debug("latest AMSC raw history overlay skipped city={}: {}", city, exc)
         return False
-    payload["runway_plate_history"] = deepcopy(existing_history) if isinstance(existing_history, dict) else {}
+    payload["runway_plate_history"] = {}
     changed = False
     for row in rows if isinstance(rows, list) else []:
         if not isinstance(row, dict):