Avoid raw runway history rebuild on hot path

tests/test_latest_observation_overlay.py

@@ -161,6 +161,53 @@ def test_overlay_builds_amsc_runway_history_from_raw_store():
     ]
 
 
+def test_overlay_skips_raw_store_when_runway_history_already_has_points():
+    class FakeDB:
+        def get_latest_raw_observation(self, source, city):
+            assert (source, city) == ("amsc_awos", "chengdu")
+            return {
+                "observed_at": "2026-06-15T11:08:00+00:00",
+                "station_code": "ZUUU",
+                "payload": {
+                    "source": "amsc_awos",
+                    "icao": "ZUUU",
+                    "temp_c": 25.4,
+                    "observation_time": "2026-06-15T11:08:00+00:00",
+                    "runway_obs": {
+                        "point_temperatures": [
+                            {
+                                "runway": "02L/20R",
+                                "target_runway_max": 25.4,
+                            },
+                        ],
+                    },
+                },
+            }
+
+        def list_raw_observation_history(self, source, city, *, minutes=60, limit=1000):
+            raise AssertionError("raw store history should not be read for populated runway history")
+
+    result = overlay_latest_amsc_observation(
+        FakeDB(),
+        "chengdu",
+        {
+            "name": "chengdu",
+            "temp_symbol": "°C",
+            "runway_plate_history": {
+                "02L/20R": [
+                    {"time": "2026-06-15T11:04:00+00:00", "temp": 25.6},
+                    {"time": "2026-06-15T11:06:00+00:00", "temp": 25.5},
+                ],
+            },
+        },
+    )
+
+    assert result["runway_plate_history"]["02L/20R"][-1] == {
+        "time": "2026-06-15T11:08:00+00:00",
+        "temp": 25.4,
+    }
+
+
 def test_overlay_uses_latest_success_when_newer_status_row_has_no_observation():
     class FakeDB:
         def get_latest_raw_observation(self, source, city):

web/services/latest_observation_overlay.py

@@ -313,6 +313,16 @@ def _append_amsc_runway_history_from_raw_store(
     city: str,
     payload: dict[str, Any],
 ) -> bool:
+    existing_history = payload.get("runway_plate_history")
+    if isinstance(existing_history, dict):
+        existing_points = [
+            len(points)
+            for points in existing_history.values()
+            if isinstance(points, list)
+        ]
+        if max(existing_points or [0]) > 1:
+            return False
+
     lister = getattr(db, "list_raw_observation_history", None)
     if not callable(lister):
         return False
@@ -321,7 +331,6 @@ 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
-    existing_history = payload.get("runway_plate_history")
     payload["runway_plate_history"] = deepcopy(existing_history) if isinstance(existing_history, dict) else {}
     changed = False
     for row in rows if isinstance(rows, list) else []: