fix(mpc-service): add backup service environment variables

Add BACKUP_SERVICE_URL, BACKUP_SERVICE_ENABLED, and SERVICE_JWT_SECRET
to mpc-service configuration to enable backup share storage.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/docker-compose.yml

@@ -359,6 +359,10 @@ services:
       # Blockchain Service - 使用正确的容器名和端口
       - BLOCKCHAIN_SERVICE_URL=http://rwa-blockchain-service:3012
       - SHARE_MASTER_KEY=${SHARE_MASTER_KEY}
+      # Backup Service
+      - BACKUP_SERVICE_URL=http://backup-service:3002
+      - BACKUP_SERVICE_ENABLED=true
+      - SERVICE_JWT_SECRET=${SERVICE_JWT_SECRET}
     depends_on:
       postgres:
         condition: service_healthy