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/providers/nws/time.go

@@ -25,3 +25,26 @@ func ParseTime(s string) (time.Time, error) {
 
 	return time.Time{}, fmt.Errorf("unsupported NWS timestamp format: %q", s)
 }
+
+// ParseTimeBestEffort parses an NWS timestamp and returns it in UTC.
+//
+// This is a convenience for normalizers that want "best effort" parsing:
+// invalid/empty strings do not fail the entire normalization; they return zero time.
+func ParseTimeBestEffort(s string) time.Time {
+	t, err := ParseTime(s)
+	if err != nil {
+		return time.Time{}
+	}
+	return t.UTC()
+}
+
+// ParseTimePtr parses an NWS timestamp and returns a UTC *time.Time.
+//
+// Empty/unparseable input returns nil. This is useful for optional CAP fields.
+func ParseTimePtr(s string) *time.Time {
+	t := ParseTimeBestEffort(s)
+	if t.IsZero() {
+		return nil
+	}
+	return &t
+}