feat(rds-refresh): add DBLab config update and remove Lambda code

- Add UpdateSourceConfig method to update DBLab source connection
  before triggering refresh, ensuring DBLab knows the clone endpoint
- Add source database credentials (dbName, username, password) to
  config for DBLab connection configuration
- Remove Lambda-specific code, focusing on container/CLI deployment
  which better suits the long-running nature of refresh operations
- Remove duplicate engine-integrated code (engine/internal/rdsrefresh
  and engine/cmd/rds-refresh) since standalone component is preferred
- Update documentation with container deployment options (Docker,
  ECS Task, Kubernetes CronJob, CLI with cron)
- Update workflow to include config update step before refresh trigger

Author: claude