Promotes or demotes a project-group member: pass the user's ID in the body together with isManager: true to promote them, or isManager: false to demote them. The user must already belong to the project group and must currently be in the opposite state. Use this to delegate a project group's day-to-day administration without touching the user's organisation-level role.

Note: if the target user is already in the requested state (e.g. promoting an existing manager, or demoting a regular member), the call returns 400 (Bad Request) rather than the more accurate 409 Conflict — handle 400 here as a possible already-in-state signal.