Add `fix:version-related-ids` ace command to backfill and correct
related_document_id on IsNewVersionOf / IsPreviousVersionOf references,
resolving the target dataset via its DOI. Handles both NULL and
self-referential (wrong) values that the existing detect command could
not repair.
Make the dataset version-chain API DOI-based: resolve previous/newer
versions through the DOI in the reference value instead of the
unreliable related_document_id, so the chain is correct regardless of
the stored FK.
