Refactor normalizers: dedupe JSON decode + event finalize

Add shared normalizer helpers to centralize payload extraction, JSON decoding,
and event finalization/validation.

Refactor NWS, Open-Meteo, and OpenWeather observation normalizers to use the
shared spine, removing repeated boilerplate while preserving provider-specific
mapping logic.

internal/normalizers/nws/observation.go

@@ -3,7 +3,6 @@ package nws
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 	"strings"
 	"time"
@@ -34,35 +33,12 @@ func (ObservationNormalizer) Match(e event.Event) bool {
 func (ObservationNormalizer) Normalize(ctx context.Context, in event.Event) (*event.Event, error) {
 	_ = ctx // normalization is pure/CPU; keep ctx for future expensive steps
 
-	rawBytes, err := normcommon.PayloadBytes(in)
-	if err != nil {
-		return nil, fmt.Errorf("nws observation normalize: %w", err)
-	}
-
-	var parsed nwsObservationResponse
-	if err := json.Unmarshal(rawBytes, &parsed); err != nil {
-		return nil, fmt.Errorf("nws observation normalize: decode raw payload: %w", err)
-	}
-
-	obs, effectiveAt, err := buildObservation(parsed)
-	if err != nil {
-		return nil, err
-	}
-
-	out := in
-	out.Schema = standards.SchemaWeatherObservationV1
-	out.Payload = obs
-
-	// EffectiveAt is optional; for observations it is naturally the observation timestamp.
-	if !effectiveAt.IsZero() {
-		t := effectiveAt.UTC()
-		out.EffectiveAt = &t
-	}
-
-	if err := out.Validate(); err != nil {
-		return nil, err
-	}
-	return &out, nil
+	return normcommon.NormalizeJSON(
+		in,
+		"nws observation",
+		standards.SchemaWeatherObservationV1,
+		buildObservation,
+	)
 }
 
 // buildObservation contains the domain mapping logic (provider -> canonical model).