fix: use actual particle child particle creation time

This was causing issues in which the last_child_created_at field for
stream particles was slightly after the actual particle's creation time,
and so comparing whether it's unseen merely via the playback markers
would be inaccurate.

go/cmd/particleprocessorworker/main.go

@@ -102,7 +102,7 @@ func main() {
 
 			// --- Perform side effects ---
 
-			updateParentLastChildCreatedAt(ctx, change.Doc.Ref)
+			updateParentLastChildCreatedAt(ctx, change.Doc)
 			transcribeMediaParticle(ctx, depotSvc, speechSvc, change.Doc)
 
 			if err := processingRepo.MarkProcessed(ctx, particleID); err != nil {
@@ -192,16 +192,18 @@ func toFirestoreTranscript(result *speech.TranscriptResult) particle.FirestoreTr
 	}
 }
 
-// updateParentLastChildCreatedAt updates the parent particle's field only if it's a particle of type stream
+// updateParentLastChildCreatedAt updates the parent stream's last_child_created_at
-func updateParentLastChildCreatedAt(ctx context.Context, docRef *firestore.DocumentRef) {
+// to the child's actual created_at timestamp, so it stays directly comparable with
-	parentChildrenCollectionRef := docRef.Parent
+// playback markers (which also store child created_at values).
+func updateParentLastChildCreatedAt(ctx context.Context, doc *firestore.DocumentSnapshot) {
+	parentChildrenCollectionRef := doc.Ref.Parent
 	if parentChildrenCollectionRef == nil {
 		return
 	}
 
 	parentParticleDocRef := parentChildrenCollectionRef.Parent
 	if parentParticleDocRef == nil {
-		slog.Error("particle has no parent document", "particleID", docRef.ID)
+		slog.Error("particle has no parent document", "particleID", doc.Ref.ID)
 		return
 	}
 
@@ -225,16 +227,25 @@ func updateParentLastChildCreatedAt(ctx context.Context, docRef *firestore.Docum
 		return
 	}
 
-	if particleType == particle.TypeStream {
+	if particleType != particle.TypeStream {
-		slog.Info("going to update the last_child_created_at for parent particle")
+		return
-		_, err = parentParticleDocRef.Update(ctx, []firestore.Update{
+	}
-			{
+
-				Path:  "last_child_created_at",
+	// Read the child's created_at — this is the same value that playback markers store
-				Value: firestore.ServerTimestamp,
+	childCreatedAt, err := doc.DataAt("created_at")
-			},
+	if err != nil {
-		})
+		slog.Error("failed to read child created_at", "error", err, "particleID", doc.Ref.ID)
-		if err != nil {
+		return
-			slog.Error("unable to update parent particle `last_child_created_at`")
+	}
-		}
+
+	slog.Info("going to update the last_child_created_at for parent particle")
+	_, err = parentParticleDocRef.Update(ctx, []firestore.Update{
+		{
+			Path:  "last_child_created_at",
+			Value: childCreatedAt,
+		},
+	})
+	if err != nil {
+		slog.Error("unable to update parent particle `last_child_created_at`")
 	}
 }