Searches across a combined dataset of 15 items (5 users + 5 products + 5 books).

query     - Search text (matches name/title)
category  - Filter: users, products, or books
sort      - asc (default) or desc
page      - Page number (default: 1)
limit     - Items per page (default: 5, max: 50)

{
  "data": [{"name": "...", "category": "users", ...}, ...],
  "meta": {"page": 1, "per_page": 5, "total": 15, "last_page": 3}
}