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/openweather/common.go

@@ -4,6 +4,8 @@ package openweather
 import (
 	"fmt"
 	"strings"
+
+	normcommon "gitea.maximumdirect.net/ejr/weatherfeeder/internal/normalizers/common"
 )
 
 // This file holds provider-specific helpers that are shared across multiple
@@ -67,5 +69,5 @@ func openWeatherStationID(parsed owmResponse) string {
 		return fmt.Sprintf("OPENWEATHER(%d)", parsed.ID)
 	}
 	// Fallback: synthesize from coordinates.
-	return fmt.Sprintf("OPENWEATHER(%.5f,%.5f)", parsed.Coord.Lat, parsed.Coord.Lon)
+	return normcommon.SynthStationID("OPENWEATHER", parsed.Coord.Lat, parsed.Coord.Lon)
 }