Notify firm when an employee leaves (fired / resigned)

Notify online firm members in-game when an employee leaves — either fired by management or resigning.

Built on the firm notification system (PAR-94).

Technical notes — departures run through StaffCommands.fire / StaffCommands.resign → FirmStaffService.fireEmployee / resignFromFirm. After the staff row is closed, notifyFirm(firmId, "business.notify.employee.left"/"business.notify.employee.resigned", "player", name). Capture recipients before the firm membership changes if the leaver should/shouldn't be included (resign: arguably notify the rest; fire: the fired player already gets the existing direct message).