Wire BigQuery core_api.events ↔ users join for true ghost-rate delta

Today 'ghost' (never-active) rate is 62.5% of all users, but this is computed only from users.last_active which is fed by extension heartbeats. core_api.events has anonymous extension events without a user_id at the JSON root — so we can't join to confirm whether ghosts are truly inactive or just have a broken heartbeat path. Action: surface the user_id from core_api.events (likely in a properties subfield) and persist it during ETL, then join to users.id for a real ghost rate. Until then ghost % is a proxy only.