ALL-957: Relay Conferences attendance-planning backend scope

Linear ALL-957: Relay Conferences attendance-planning surface. Sequenced SECOND after Lyra ALL-956 RBM entity v1 lands. Scope backend only: schema/migration for planning status enum (attending/not_attending/TBD plus existing target buckets if retained), Texture attendee many-to-many/addition-order support, internal_note, RBM FK + nrtc_org_wide flag once ALL-956 contract is available; GraphQL filters/mutations/computed nextConference/counts. Keep frontend/UI in separate Atlas PRs. Project context: projects/relay-conferences.md. Branch planned: talos/all-957-conference-attendance-backend. Do not start RBM-dependent implementation until ALL-956 is merged/deployed or contract explicitly stabilized.