Sender health/registration poll (§3.3).

A non-401 response confirms the operator key is valid + active. Returns the open-mission queue depth (work the sender still has to act on), mirroring the /sync/missions status set, plus the tenant's current auto_threshold so the sender knows the gate without a second call. version is null until the sender reports its own (W3/W4); _auth already bumped last_seen_at.