Get Sending Identity

T-41 PR-4 — read-only sender visibility for the "Sending from" UI.

Which identity this org's campaigns send from: its own sending domains (with warming/active status) once it owns any, and the sender names on its active dedicated mailboxes. pool='dedicated' uses the same predicate as the send-side picker gate (api.outreach.db.org_has_active_dedicated_mailboxes) so the badge can never claim an identity the picker wouldn't use; an org whose domains are still warming stays pool='shared'.

A shared-pool org (every org until the T-41 BYO onboarding ships) gets the empty shell — the operator fleet's domains/mailboxes must never leak through this endpoint.

T-41 rollout switch: while BYO rolls out, orgs not listed in OUTREACH_BYO_ENABLED_ORGS (outreach_db.byo_org_enabled — the same switch that short-circuits the picker gate) get the empty shell no matter what rows exist, so the UI stays exactly current-prod for every real user.