In standalone mode, mpc-service runs on the 1.0 server (192.168.1.111),
not in the local Docker network. The default docker-compose value
http://mpc-service:3006 won't resolve. Export MPC_SERVICE_URL pointing
to the 1.0 server's IP so mining-blockchain-service can reach it.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>