Extend preprint-publication dedup from FLoRA to FReD

## Context

FLoRA now has preprint-publication deduplication (added for #105) that:
- Detects when the same paper appears as both preprint and published version (different DOIs)
- Resolves confirmed duplicates, keeping the published version and storing the alternative DOI in `doi_o_alt` / `doi_r_alt` columns
- Handles both replication-side (same `doi_o`, different `doi_r`) and original-side (different `doi_o` for the same paper) duplicates

## What needs to happen

1. **Extend dedup to FReD**: The same preprint-publication detection logic (`R/preprint_dedup.R`) should be applied to the FReD effect-level dataset, not just the paper-level FLoRA dataset.

2. **Add alternative DOI columns even without duplicates**: Where FReD references a DOI that has a known preprint/published counterpart (from the FLoRA confirmed duplicates or CrossRef metadata), `doi_o_alt` and `doi_r_alt` should be populated even if FReD only contains one version. This ensures users can look up papers by either DOI.

## References

- Preprint dedup logic: `R/preprint_dedup.R`
- FLoRA pipeline integration: Step 7c in `pipelines/flora/prepare_flora.qmd`
- Confirmed duplicates: `cache/confirmed_preprint_duplicates.csv`
- Original issue: #105