Measure product demand

Grouping after `$unwind` changes the grain from orders to products. Accumulators can then measure both physical demand and money from the same line-item stream.

Using completed orders, return `productId`, total `units`, and `revenue` for every product. Sort by revenue descending, then product ID ascending.

Expected result: One clean demand summary per sold product.

[
  { $match: { status: "completed" } },
  { $unwind: "$items" }
]