Backups and restores fail on the workers: No module named 'fastapi' #660

Backups, restores and the six-hourly backup cleanup all fail on the workers with ModuleNotFoundError: No module named 'fastapi'.

create_backup_task, restore_backup_task and cleanup_backup_artifacts in backend/worker/tasks/system.py import backend.core.backup, whose restore stages import backend/utils/document_storage.py, which imports HTTPException from FastAPI at module level. The worker images install requirements/worker.txt, which has no FastAPI. The chain arrived with #571 (22 Oct), so no release has it. CI installs both requirement sets together, so the suite never saw it.

Fix: keep FastAPI out of modules the workers import, and add a test that imports every worker task module with FastAPI unavailable.

Ours