ALL-991: Conferences resolvers + federation

Implement ALL-957/B backend resolver/federation slice after ALL-990 schema work: status/planning status, attendees, internal_note, filtered query/computed counts. Defer RBM resolver pieces until Lyra ALL-956 RBM contract lands. Source #agent-ops pm-driver nudge 1779452112.590769. Branch/pr boundary: talos/all-990-conferences-schema-data-model unless split becomes necessary.