S3 object versions may share one chunk list, so deleting the old chunks
after a repack could corrupt sibling versions; reject those entries
until chunk ownership is tracked.
New chunks also carried the full original TTL, restarting needle
expiry at repack time while entry expiry stayed anchored to creation: a
nearly expired entry left chunks stored for almost a full extra span.
Assign the remaining lifetime instead, and reject entries already past
it.