Target multiple roles in a single campaign

Currently, campaigns can only be targeted to one role at a time — 

the role filter in the dashboard allows a single selection, and the 

API accepts UIDs rather than roles. Customers with role-based user 

segments (e.g. Admin, Employee, Customer, Supplier) need to create 

and send a separate campaign for each role, which is repetitive and 

hard to manage at scale. Adding multi-role selection so a single 

campaign can target a combination of roles (e.g. Admin + Customer) 

would let customers reach the right audience in one send.