Get Attribution Summary

Per-channel/campaign return rates, plus the baseline they should be read against.

Adjudicated sends only — pending sends (window still open, or event coverage lagging) are excluded from every denominator.

organic_baseline is the share of quiet stretches that end in a return with NO message sent, computed from this workspace's own events. It is null when the workspace has no baseline yet, in which case callers must show no comparison rather than inventing one: a return rate on its own cannot tell you whether the message did anything.