- Add mysql:8.0 service with health check and named volume for data persistence
- Backend depends_on db with service_healthy condition
- Override SD_DB_HOST=db so backend connects to MySQL container via Docker network
- Rename backend container and image to superdreamfront
- Update .env.example to note "db" host for docker-compose usage
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>