Link experiments to a specified experiment

Adds one or more experiments as related entries on the source experiment so they appear under GET /experiments/{experimentID}/links. Send the target experiment IDs as a JSON array in the body, e.g. [42, 51].

Links are bidirectional — once created, the source experiment also shows up under each target's links, and an audit-log entry is written on both sides. The caller must have access to the source and to every target; an inaccessible target rejects the whole call. Linking an experiment to itself is rejected.

Note: if any target is already linked to the source, the call returns 401 (Unauthorized) rather than the more accurate 409 Conflict — handle 401 from this endpoint as a possible duplicate-link signal, not just an auth failure.