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/id.go

@@ -0,0 +1,39 @@
+// FILE: ./internal/sources/common/id.go
+package common
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ChooseEventID applies weatherfeeder's opinionated Event.ID policy:
+//
+//   - If upstream provides an ID, use it (trimmed).
+//   - Otherwise, ID is "<Source>:<EffectiveAt>" when available.
+//   - If EffectiveAt is unavailable, fall back to "<Source>:<EmittedAt>".
+//
+// Timestamps are encoded as RFC3339Nano in UTC.
+func ChooseEventID(upstreamID, sourceName string, effectiveAt *time.Time, emittedAt time.Time) string {
+	if id := strings.TrimSpace(upstreamID); id != "" {
+		return id
+	}
+
+	src := strings.TrimSpace(sourceName)
+	if src == "" {
+		src = "UNKNOWN_SOURCE"
+	}
+
+	// Prefer EffectiveAt for dedupe friendliness.
+	if effectiveAt != nil && !effectiveAt.IsZero() {
+		return fmt.Sprintf("%s:%s", src, effectiveAt.UTC().Format(time.RFC3339Nano))
+	}
+
+	// Fall back to EmittedAt (still stable within a poll invocation).
+	t := emittedAt.UTC()
+	if t.IsZero() {
+		t = time.Now().UTC()
+	}
+
+	return fmt.Sprintf("%s:%s", src, t.Format(time.RFC3339Nano))
+}