fix(webhook): skip review webhooks without reviewer id or timestamp

Guard pull_request_review submissions that lack review.user.id or
review.submitted_at, matching the backfill path in github-fetcher.service.ts.
Prevents webhook 500s and NOT NULL upsert failures on ghost reviewers.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: jason020818

packages/das/src/webhook/handlers/review.handler.ts

@@ -20,6 +20,9 @@ export class ReviewHandler {
     // Only store submitted reviews (not pending/dismissed)
     if (payload.action !== "submitted") return;
 
+    // Mirror backfill guard: skip reviews without a reviewer id or timestamp.
+    if (!review?.user?.id || !review?.submitted_at) return;
+
     const data: Partial<Review> = {
       repoFullName,
       prNumber: payload.pull_request.number,