Issue #108 Milestone 1. Adds BigQuery table registration via /admin/tables UI and `da admin register-table` CLI without hand-editing table_registry. POST /api/admin/register-table/precheck for round-trip validation. --dry-run flag on CLI. Audit-log entries on register/update/unregister. PUT /api/admin/registry/{id} now preserves registered_at (closes#130).