Unlink experiments from a specified experiment

Removes the bidirectional link between the source experiment and each of the supplied target experiments — pass the target IDs as a JSON array in the body. After this call, neither side lists the other under GET /experiments/{experimentID}/links, and an audit-log entry is written on both sides.

The caller must have access to the source and to every target. Unlinking an experiment from itself is rejected.

Note: if any target is not currently linked to the source, the call returns 401 (Unauthorized) rather than 404 Not Found or 409 Conflict — handle 401 from this endpoint as a possible already-unlinked signal, not just an auth failure.