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/common.go

@@ -1,19 +0,0 @@
-// FILE: ./internal/normalizers/openmeteo/common.go
-package openmeteo
-
-import (
-	"time"
-
-	openmeteo "gitea.maximumdirect.net/ejr/weatherfeeder/internal/providers/openmeteo"
-)
-
-// parseOpenMeteoTime parses Open-Meteo timestamps.
-//
-// The actual parsing logic lives in internal/providers/openmeteo so both the
-// source (envelope EffectiveAt / event ID) and normalizer (canonical payload)
-// can share identical timestamp behavior.
-//
-// We keep this thin wrapper to avoid churn in the normalizer package.
-func parseOpenMeteoTime(s string, tz string, utcOffsetSeconds int) (time.Time, error) {
-	return openmeteo.ParseTime(s, tz, utcOffsetSeconds)
-}

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,