feat(docker-compose): add PARTY_ID to server-party-api configuration

- Add explicit PARTY_ID environment variable for delegate party
- Set PARTY_ID=delegate-party for server-party-api service
- This ensures the delegate party properly registers to Message Router party pool
- Enables delegate party selection for keygen sessions with require_delegate=true

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

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

backend/mpc-system/docker-compose.yml

@@ -252,6 +252,8 @@ services:
       MPC_CRYPTO_MASTER_KEY: ${CRYPTO_MASTER_KEY}
       # API 认证密钥 (与 mpc-service 配置的 MPC_API_KEY 一致)
       MPC_API_KEY: ${MPC_API_KEY}
+      # Party identity for Message Router registration
+      PARTY_ID: delegate-party
     depends_on:
       session-coordinator:
         condition: service_healthy