fix(docker-compose): add ACCOUNT_SERVICE_ADDR to session-coordinator

- Add ACCOUNT_SERVICE_ADDR environment variable pointing to account-service:8080
- Fixes "connection refused" error when session-coordinator tries to auto-create account after keygen
- Session-coordinator can now properly call account service to create account records

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

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

backend/mpc-system/docker-compose.yml

@@ -66,6 +66,7 @@ services:
       MPC_JWT_SECRET_KEY: ${JWT_SECRET_KEY}
       MPC_JWT_ISSUER: mpc-system
       MESSAGE_ROUTER_ADDR: message-router:50051
+      ACCOUNT_SERVICE_ADDR: http://account-service:8080
     depends_on:
       postgres:
         condition: service_healthy