Deletes a customer.

Requires ManageCustomers and an Idempotency-Key header. This is the same soft delete as the Customers page: the customer disappears from every list and lookup (GETs answer 404) but keeps its history, and a user can restore it from Customers → Deleted Customers in the app. The delete is refused with 409 while the customer still has active jobs (not Completed/Cancelled), unpaid invoices (not Paid/Cancelled) or active sales orders — complete or cancel those first; the problem detail names which. To merely hide a customer from pickers without deleting it, PUT it with isActive: false instead.