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/common/event.go

@@ -9,17 +9,34 @@ import (
 
 // SingleRawEvent constructs, validates, and returns a slice containing exactly one event.
 //
-// This removes the repetitive "event envelope ceremony" from individual sources.
+// This removes repetitive "event envelope ceremony" from individual sources.
 // Sources remain responsible for:
 //   - fetching bytes (raw payload)
 //   - choosing Schema (raw schema identifier)
-//   - computing a stable Event.ID and (optional) EffectiveAt
-func SingleRawEvent(kind event.Kind, sourceName string, schema string, id string, effectiveAt *time.Time, payload any) ([]event.Event, error) {
+//   - computing Event.ID and (optional) EffectiveAt
+//
+// emittedAt is explicit so callers can compute IDs using the same timestamp (or
+// so tests can provide a stable value).
+func SingleRawEvent(
+	kind event.Kind,
+	sourceName string,
+	schema string,
+	id string,
+	emittedAt time.Time,
+	effectiveAt *time.Time,
+	payload any,
+) ([]event.Event, error) {
+	if emittedAt.IsZero() {
+		emittedAt = time.Now().UTC()
+	} else {
+		emittedAt = emittedAt.UTC()
+	}
+
 	e := event.Event{
 		ID:          id,
 		Kind:        kind,
 		Source:      sourceName,
-		EmittedAt:   time.Now().UTC(),
+		EmittedAt:   emittedAt,
 		EffectiveAt: effectiveAt,
 
 		// RAW schema (normalizer matches on this).