Remove Kafka event-driven MPC signing (publish/consume pattern) and replace
with HTTP POST + polling to mpc-service, matching identity-service approach.
- Rewrite mpc-signing.client.ts: POST /api/v1/mpc/sign + poll status
- Delete mpc-event-consumer.service.ts (no longer needed)
- Delete mpc-keygen-completed.handler.ts (keygen handled elsewhere)
- Add MPC_SERVICE_URL env var to docker-compose.2.0.yml
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>