Fix optional-discharge crashes and dedupe measurement mapping

- MeasurementResponse.discharge is now Optional[float]; measurements with a
  null discharge no longer raise a Pydantic ValidationError (HTTP 500) on the
  /measurements/latest and /measurements/station endpoints.
- InfluxDB save_measurements guards float(discharge) against None instead of
  crashing with TypeError.
- Extract the duplicated measurement->response mapping into a single
  _to_measurement_response helper used by both measurement endpoints.

src/database_adapters.py

@@ -111,9 +111,11 @@ class InfluxDBAdapter(DatabaseAdapter):
                     "time": measurement["timestamp"].isoformat(),
                     "fields": {
                         "water_level": float(measurement["water_level"]),
-                        "discharge": float(measurement["discharge"]),
+                        "discharge": float(measurement["discharge"])
+                        if measurement.get("discharge") is not None
+                        else None,
                         "discharge_percent": float(measurement["discharge_percent"])
-                        if measurement["discharge_percent"]
+                        if measurement.get("discharge_percent")
                         else None,
                     },
                 }