Delete a category

What your assistant is told

Delete one of the user's OWN custom categories. category_id is the category's UUID (from list_categories). Guardrails enforced by the service, each surfaced as a clear error: system-default categories cannot be deleted, and a category anything still references cannot be deleted — a transaction (recategorize or delete those first, see update_transaction), a monthly budget cap on it (see get_budgets), or another category naming it as parent. All three come back as category_in_use; the error message says which one, so clear that and retry. This cannot be undone. Returns the deleted id.