Fix periodic poll never finding new readings

extract_readings() only read the latestReading field embedded in the
stations-endpoint response. For some stations that field is always
null even though the measure's own /readings endpoint has current
data, so every non-backfill poll silently found 0 readings forever.
Now falls back to fetching the single latest reading directly from
the measure when the embedded field is missing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

river_monitor.py

@@ -160,7 +160,22 @@ def fetch_historical_readings(
     return readings
 
 
-def extract_readings(stations: Iterable[dict]) -> list[Reading]:
+def fetch_latest_reading(session: requests.Session, measure_id: str) -> Optional[dict]:
+    """Fetch a measure's single most recent reading directly. Some stations
+    never populate the stations-endpoint's embedded `latestReading` field,
+    even though the measure's own /readings endpoint has current data."""
+    try:
+        resp = session.get(
+            f"{measure_id}/readings", params={"_sorted": "", "_limit": 1}, timeout=15
+        )
+        resp.raise_for_status()
+    except requests.RequestException:
+        return None
+    items = resp.json().get("items", [])
+    return items[0] if items else None
+
+
+def extract_readings(session: requests.Session, stations: Iterable[dict]) -> list[Reading]:
     readings: list[Reading] = []
     for station in stations:
         station_id = station.get("stationReference") or station.get("@id", "").rsplit("/", 1)[-1]
@@ -178,7 +193,10 @@ def extract_readings(stations: Iterable[dict]) -> list[Reading]:
         for measure in measures:
             if measure.get("parameter") != "level":
                 continue
+            measure_id = measure.get("@id", "")
             latest = measure.get("latestReading")
+            if not latest and measure_id:
+                latest = fetch_latest_reading(session, measure_id)
             if not latest:
                 continue
             readings.append(
@@ -188,7 +206,7 @@ def extract_readings(stations: Iterable[dict]) -> list[Reading]:
                     river_name=river_name,
                     lat=lat,
                     lon=lon,
-                    measure_id=measure.get("@id", ""),
+                    measure_id=measure_id,
                     unit=measure.get("unitName"),
                     qualifier=measure.get("qualifier"),
                     value=latest.get("value"),
@@ -445,7 +463,7 @@ def poll_once(cfg: dict, session: requests.Session, verbose: bool = True, backfi
         print(f"[{ts}] Backfilling {backfill_days} days of history for {len(stations)} stations...")
         readings = fetch_historical_readings(session, stations, backfill_days)
     else:
-        readings = extract_readings(stations)
+        readings = extract_readings(session, stations)
     new_rows = store_readings(conn, readings)
     if not no_html:
         build_dashboard(conn, Path(cfg["output"]))