Add an existing customer as an employee

Add an address to a company POST

Adds an address. Setting `is_default` clears the default flag on the company's other addresses of the same type.

Delete a company DELETE

Soft-deletes the company and its addresses, sets every employee to `removed`, and deletes its customer group (price lists and promotions that targeted the group stop applying). Customers stay as plain customers. Orders keep their company link for history. Emits `employee.removed` per employee, then `company.deleted`.