sources: standardize Event.ID on Source:EffectiveAt; simplify raw event helper

- Adopt an opinionated Event.ID policy across sources:
  - use upstream-provided ID when available
  - otherwise derive a stable ID from Source:EffectiveAt (RFC3339Nano, UTC)
  - fall back to Source:EmittedAt when EffectiveAt is unavailable
- Add common/id helper to centralize ID selection logic and keep sources consistent
- Simplify common event construction by collapsing SingleRawEventAt/SingleRawEvent
  into a single explicit SingleRawEvent helper (emittedAt passed in)
- Update NWS/Open-Meteo/OpenWeather observation sources to:
  - compute EffectiveAt first
  - generate IDs via the shared helper
  - build envelopes via the unified SingleRawEvent helper
- Improve determinism and dedupe-friendliness without changing schemas or payloads

internal/sources/openweather/observation.go

@@ -15,12 +15,6 @@ import (
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/standards"
 )
 
-// ObservationSource polls the OpenWeatherMap "Current weather" endpoint and emits a RAW observation Event.
-//
-// IMPORTANT UNIT POLICY (weatherfeeder convention):
-// OpenWeather changes units based on the `units` query parameter but does NOT include the unit
-// system in the response body. To keep normalization deterministic, this driver *requires*
-// `units=metric`. If absent (or non-metric), the driver returns an error.
 type ObservationSource struct {
 	http *common.HTTPSource
 }
@@ -45,7 +39,6 @@ func (s *ObservationSource) Name() string { return s.http.Name }
 func (s *ObservationSource) Kind() event.Kind { return event.Kind("observation") }
 
 func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
-	// Re-check policy defensively (in case the URL is mutated after construction).
 	if err := requireMetricUnits(s.http.URL); err != nil {
 		return nil, fmt.Errorf("%s %q: %w", s.http.Driver, s.http.Name, err)
 	}
@@ -55,22 +48,21 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 		return nil, err
 	}
 
-	eventID := buildEventID(s.http.Name, meta)
-	if strings.TrimSpace(eventID) == "" {
-		eventID = fmt.Sprintf("openweather:current:%s:%s", s.http.Name, time.Now().UTC().Format(time.RFC3339Nano))
-	}
-
 	var effectiveAt *time.Time
 	if !meta.ParsedTimestamp.IsZero() {
 		t := meta.ParsedTimestamp.UTC()
 		effectiveAt = &t
 	}
 
+	emittedAt := time.Now().UTC()
+	eventID := common.ChooseEventID("", s.http.Name, effectiveAt, emittedAt)
+
 	return common.SingleRawEvent(
 		s.Kind(),
 		s.http.Name,
 		standards.SchemaRawOpenWeatherCurrentV1,
 		eventID,
+		emittedAt,
 		effectiveAt,
 		raw,
 	)
@@ -81,14 +73,6 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 type openWeatherMeta struct {
 	Dt int64 `json:"dt"` // unix seconds, UTC
 
-	ID   int64  `json:"id"`
-	Name string `json:"name"`
-
-	Coord struct {
-		Lon float64 `json:"lon"`
-		Lat float64 `json:"lat"`
-	} `json:"coord"`
-
 	ParsedTimestamp time.Time `json:"-"`
 }
 
@@ -110,24 +94,6 @@ func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, open
 	return raw, meta, nil
 }
 
-func buildEventID(sourceName string, meta openWeatherMeta) string {
-	locKey := ""
-	if meta.ID != 0 {
-		locKey = fmt.Sprintf("city:%d", meta.ID)
-	} else if meta.Coord.Lat != 0 || meta.Coord.Lon != 0 {
-		locKey = fmt.Sprintf("coord:%.5f,%.5f", meta.Coord.Lat, meta.Coord.Lon)
-	} else {
-		locKey = "loc:unknown"
-	}
-
-	ts := meta.ParsedTimestamp
-	if ts.IsZero() {
-		ts = time.Now().UTC()
-	}
-
-	return fmt.Sprintf("openweather:current:%s:%s:%s", sourceName, locKey, ts.Format(time.RFC3339Nano))
-}
-
 func requireMetricUnits(rawURL string) error {
 	u, err := url.Parse(strings.TrimSpace(rawURL))
 	if err != nil {