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/common/json.go

@@ -0,0 +1,68 @@
+// FILE: ./internal/normalizers/common/json.go
+package common
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// DecodeJSONPayload extracts the event payload as bytes and unmarshals it into T.
+//
+// This is the shared "spine" used by many normalizers:
+//   - sources emit raw JSON payloads (typically json.RawMessage)
+//   - normalizers decode into provider structs
+//
+// Errors include a small amount of stage context ("extract payload", "decode raw payload").
+// Callers typically wrap these with a provider/kind label.
+func DecodeJSONPayload[T any](in event.Event) (T, error) {
+	var zero T
+
+	b, err := PayloadBytes(in)
+	if err != nil {
+		return zero, fmt.Errorf("extract payload: %w", err)
+	}
+
+	var parsed T
+	if err := json.Unmarshal(b, &parsed); err != nil {
+		return zero, fmt.Errorf("decode raw payload: %w", err)
+	}
+
+	return parsed, nil
+}
+
+// NormalizeJSON is a convenience wrapper for the common JSON-normalizer pattern:
+//
+//  1. Decode raw JSON payload into provider struct T
+//  2. Map T into canonical payload P (plus an EffectiveAt timestamp)
+//  3. Finalize the event envelope (schema/payload/effectiveAt) + Validate
+//
+// label should be short and specific, e.g. "openweather observation".
+// outSchema should be the canonical schema constant.
+// build should contain ONLY provider/domain mapping logic.
+func NormalizeJSON[T any, P any](
+	in event.Event,
+	label string,
+	outSchema string,
+	build func(parsed T) (P, time.Time, error),
+) (*event.Event, error) {
+	parsed, err := DecodeJSONPayload[T](in)
+	if err != nil {
+		return nil, fmt.Errorf("%s normalize: %w", label, err)
+	}
+
+	payload, effectiveAt, err := build(parsed)
+	if err != nil {
+		// build() should already include provider-specific context where appropriate.
+		return nil, err
+	}
+
+	out, err := Finalize(in, outSchema, payload, effectiveAt)
+	if err != nil {
+		return nil, fmt.Errorf("%s normalize: %w", label, err)
+	}
+
+	return out, nil
+}