fix(mpc-service): add BLOCKCHAIN_SERVICE_URL config

Configure correct blockchain-service URL (port 3012) for address derivation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/services/docker-compose.yml

@@ -343,6 +343,8 @@ services:
       - MPC_SERVER_PARTY_API_URL=http://mpc-server-party-api:8080
       # JWT Secret 必须与 mpc-system 的 JWT_SECRET_KEY 一致
       - MPC_JWT_SECRET=change_this_jwt_secret_key_to_random_value_min_32_chars
+      # Blockchain Service - 使用正确的容器名和端口
+      - BLOCKCHAIN_SERVICE_URL=http://rwa-blockchain-service:3012
       - SHARE_MASTER_KEY=${SHARE_MASTER_KEY}
     depends_on:
       postgres: