refactor(normalizers): deduplicate synthetic station ID generation

- Add common SynthStationID helpers for coordinate-based providers
- Use shared helper for Open-Meteo and OpenWeather station ID synthesis
- Require both lat/lon when generating synthetic IDs to avoid misleading defaults
- Remove unused Open-Meteo normalizer wrapper code

This reduces cross-provider duplication while keeping provider-specific
mapping logic explicit and readable.

internal/normalizers/openmeteo/observation.go

@@ -78,18 +78,8 @@ func buildObservation(parsed omResponse) (model.WeatherObservation, time.Time, e
 	canonicalText := standards.WMOText(wmo, isDay)
 
 	// Station identity: Open-Meteo is not a station feed; synthesize from coordinates.
-	stationID := ""
-	if parsed.Latitude != nil || parsed.Longitude != nil {
-		lat := 0.0
-		lon := 0.0
-		if parsed.Latitude != nil {
-			lat = *parsed.Latitude
-		}
-		if parsed.Longitude != nil {
-			lon = *parsed.Longitude
-		}
-		stationID = fmt.Sprintf("OPENMETEO(%.5f,%.5f)", lat, lon)
-	}
+	// Require BOTH lat/lon to avoid misleading OPENMETEO(0.00000,...) IDs.
+	stationID := normcommon.SynthStationIDPtr("OPENMETEO", parsed.Latitude, parsed.Longitude)
 
 	obs := model.WeatherObservation{
 		StationID:   stationID,