MessageParticipantRole
: string
in package
Participant role in a thread: who started it x the other side (does not change on reply).
It has no effect on permissions: RBAC roles owner x receiver derive from the author of
individual messages (MessageThreadRolesTrait), not from this value. It serves the mailbox
("Received" / "Sent") and lets the admin form map two pickers (sender / recipient) to
participant rows and back.