fix(firebase): Only update nonOrdersTimestamp during non-orders processing runs

- Use conditional update logic to preserve nonOrdersTimestamp during orders-only runs
- Implement merge strategy to selectively update Firestore document fields
- Ensure nonOrdersTimestamp accurately reflects last non-order event processing time

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: leomp12

packages/firebase/src/handlers/check-store-events.ts

@@ -293,11 +293,14 @@ export default async () => {
       })),
     });
   }));
-  return documentRef.set({
+  const documentMergeData: Record<string, any> = {
     timestamp: maxTimestamp,
-    nonOrdersTimestamp: isOrdersOnly ? lastNonOrdersTimestamp : maxTimestamp,
     pushedTimestamps: lastPushedTimestamps,
     activeApps,
     listenedEvents,
-  });
+  };
+  if (!isOrdersOnly) {
+    documentMergeData.nonOrdersTimestamp = maxTimestamp;
+  }
+  return documentRef.set(documentMergeData, { merge: true });
 };