Fix worker start: run as module + keep health server

Force Docker Space + serve health on

Add health server so HF marks worker healthy

fix: worker deps + HF config

fix: restore worker code + entrypoint

deploy: real pdf-trainer-demo worker

sftp: add sftp_store module + copy repo in Dockerfile

fix: install worker deps via requirements.txt

fix: force long-running worker process

init: worker docker skeleton