sources: standardize HTTP source config + factor raw-event boilerplate

- Require params.user_agent for all HTTP sources (uniform config across providers)
- Add common.RequireHTTPSourceConfig() to validate name/url/user_agent in one call
- Add common.NewHTTPClient() with DefaultHTTPTimeout for consistent client setup
- Add common.SingleRawEvent() to centralize event envelope construction + validation
- Refactor NWS/Open-Meteo/OpenWeather observation sources to use new helpers

internal/sources/nws/observation.go

@@ -32,30 +32,18 @@ type ObservationSource struct {
 }
 
 func NewObservationSource(cfg config.SourceConfig) (*ObservationSource, error) {
-	if strings.TrimSpace(cfg.Name) == "" {
-		return nil, fmt.Errorf("nws_observation: name is required")
-	}
-	if cfg.Params == nil {
-		return nil, fmt.Errorf("nws_observation %q: params are required (need params.url and params.user_agent)", cfg.Name)
-	}
+	const driver = "nws_observation"
 
-	url, ok := cfg.ParamString("url", "URL")
-	if !ok {
-		return nil, fmt.Errorf("nws_observation %q: params.url is required", cfg.Name)
-	}
-
-	ua, ok := cfg.ParamString("user_agent", "userAgent")
-	if !ok {
-		return nil, fmt.Errorf("nws_observation %q: params.user_agent is required", cfg.Name)
+	c, err := common.RequireHTTPSourceConfig(driver, cfg)
+	if err != nil {
+		return nil, err
 	}
 
 	return &ObservationSource{
-		name:      cfg.Name,
-		url:       url,
-		userAgent: ua,
-		client: &http.Client{
-			Timeout: 10 * time.Second,
-		},
+		name:      c.Name,
+		url:       c.URL,
+		userAgent: c.UserAgent,
+		client:    common.NewHTTPClient(common.DefaultHTTPTimeout),
 	}, nil
 }
 
@@ -95,25 +83,14 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 		effectiveAt = &t
 	}
 
-	e := event.Event{
-		ID:          eventID,
-		Kind:        s.Kind(),
-		Source:      s.name,
-		EmittedAt:   time.Now().UTC(),
-		EffectiveAt: effectiveAt,
-
-		// RAW schema (normalizer matches on this).
-		Schema: standards.SchemaRawNWSObservationV1,
-
-		// Raw JSON; normalizer will decode and map to canonical model.WeatherObservation.
-		Payload: raw,
-	}
-
-	if err := e.Validate(); err != nil {
-		return nil, err
-	}
-
-	return []event.Event{e}, nil
+	return common.SingleRawEvent(
+		s.Kind(),
+		s.name,
+		standards.SchemaRawNWSObservationV1,
+		eventID,
+		effectiveAt,
+		raw,
+	)
 }
 
 // ---- RAW fetch + minimal metadata decode ----