Rename item catalog and add inventory JSON

Rename 05_scapegoat/06_item_catalog.md -> 03_ubisoft_services/06_inventory.md and update the document heading/content (remove the endpoint reference and add a link to a pre-made response). Add 03_ubisoft_services/inventory_all_items.json containing the full inventory response (~1,665 items) for use as a ready-made sample/test payload.