When a tenant provides a recipients authorization criterion in their Kafka or GW payload, the recipients-resolver service from Notifications will call the lookupSubjects operation from the Kessel Relations API to possibly restrict the recipients that will be notified: https://github.com/RedHatInsights/notifications-backend/blob/master/recipients-resolver/src/main/java/com/redhat/cloud/notifications/recipients/resolver/RecipientsResolver.java#L151

This is currently disabled in all environments because the Kessel Relations API is no longer supported.

The Kessel team should migrate the old lookupSubjects operation from the Relations API to the Inventory API after the M5 milestone. When that's done, we'll need to migrate our code to use the new operation from the Inventory API, then test it and enable it for customers.

Acceptance criteria:

• The lookupSubjects operation (or similar) is available in the Kessel Inventory API.
• The recipients-resolved logic is migrated away from Relations API to the new operation.
• The new code is tested in IQE.