Get adquirentes (customers) for dropdown filter

Distinct acquirers (Faturas.B) that appear in this company's faturas.

The counterpart to /emitentes. On a sales invoice the emitter is the company itself, so an emitter-only filter shows the dealer their own name and nothing else (#764 follow-up). Returns [{nif, nome}], nome falling back to "NIF: x" when the customer is not a registered Cliente.

Plain def — synchronous DB work must not run on the event loop.