Baton v0: apply Baton DB schema

Implement ALL-910. Apply Baton DB schema for baton_thread, baton_message, baton_slack_message_map, baton_automation_fire, baton_user_memory, baton_message_embedding per specs/relay/phase-3-baton/engineering-spec.md §4. Coordinate with Lyra on Neon baton-db provisioning. Branch talos/baton-db-schema.