normalizers/openweather: extract shared helpers into common.go

Refactor OpenWeather normalizers to improve structure and reuse by moving
provider-specific helper functions out of observation.go and into a new
common.go.

This keeps observation.go focused on schema matching and domain mapping,
preserves the “one normalizer per file” convention, and establishes a clear
home for helpers that will be shared by future OpenWeather forecast and alert
normalizers.

No functional behavior changes; this is a pure internal refactor.

internal/normalizers/openweather/observation.go

@@ -3,7 +3,6 @@ package openweather
 
 import (
 	"context"
-	"fmt"
 	"strings"
 	"time"
 
@@ -132,51 +131,3 @@ func buildObservation(parsed owmResponse) (model.WeatherObservation, time.Time,
 
 	return obs, ts, nil
 }
-
-func primaryCondition(list []owmWeather) (id int, desc string, icon string) {
-	if len(list) == 0 {
-		return 0, "", ""
-	}
-	w := list[0]
-	return w.ID, strings.TrimSpace(w.Description), strings.TrimSpace(w.Icon)
-}
-
-func inferIsDay(icon string, dt, sunrise, sunset int64) *bool {
-	// Prefer icon suffix.
-	icon = strings.TrimSpace(icon)
-	if icon != "" {
-		last := icon[len(icon)-1]
-		switch last {
-		case 'd':
-			v := true
-			return &v
-		case 'n':
-			v := false
-			return &v
-		}
-	}
-
-	// Fall back to sunrise/sunset bounds if provided.
-	if dt > 0 && sunrise > 0 && sunset > 0 {
-		v := dt >= sunrise && dt < sunset
-		return &v
-	}
-
-	return nil
-}
-
-func openWeatherIconURL(icon string) string {
-	icon = strings.TrimSpace(icon)
-	if icon == "" {
-		return ""
-	}
-	return fmt.Sprintf("https://openweathermap.org/img/wn/%s@2x.png", icon)
-}
-
-func openWeatherStationID(parsed owmResponse) string {
-	if parsed.ID != 0 {
-		return fmt.Sprintf("OPENWEATHER(%d)", parsed.ID)
-	}
-	// Fallback: synthesize from coordinates.
-	return fmt.Sprintf("OPENWEATHER(%.5f,%.5f)", parsed.Coord.Lat, parsed.Coord.Lon)
-}