Returns products in the category, the category's direct subcategories,
and the category itself in extra_details. Use this to walk the tree.

Notes

• A product with no variants[] is itself the variant — read price
  from the product row.
• max_modifiers: 0 means unlimited, not zero. Treat it as "no
  cap".
• Subcategories don't carry an explicit parent_id — stamp the
  category id you queried onto each subcategory as you walk the tree.
• A product can appear under multiple categories. Dedupe by id (or
  product_id) before persisting.