Assign an unrouted email

Assign a handler to an unrouted item and re-drive it. Admin-only. Sets the message’s agent (and, when the mail hit a handler-less mailbox endpoint, the endpoint’s agent too, so future mail routes), then re-emits the routed event via the outbox so the worker drives a turn. The re-drive forces the agent/VM path (plane=NULL): a manually-assigned item is rare + deliberate, and the agent path avoids the light-plane’s sender-bound disclosure ambiguity.