[DevOps]: Simplify site-ingestion launcher configuration

### Task Details

Split from #332. Support the standard deployment layout by configuring `SIMBOARD_ROOT` and deriving launcher paths. Keep explicit work/module paths only as nonstandard deployment overrides.

### Steps to Complete

- Derive work and module paths from `SIMBOARD_ROOT`.
- Resolve token-file defaults after the work directory is available.
- Update the generic crontab, scripts README, and site-onboarding guidance.
- Add launcher tests for path derivation.

### Additional Notes

- Launcher: `backend/app/scripts/ingestion/sites/site_ingestion_launcher.sh`
- Cron template: `backend/app/scripts/ingestion/sites/crontab.example`

Start with backend/app/scripts/ingestion/sites/site_ingestion_launcher.sh and backend/app/scripts/ingestion/sites/crontab.example to trace the current path and token-file configuration. Add launcher tests covering path derivation, then update the generic crontab, scripts README, and site-onboarding guidance so standard deployments use SIMBOARD_ROOT while explicit paths remain nonstandard overrides.

Written by the indexing model from the issue text.