ACATS status events on the admin-actions SSE stream

Broker API

Adds the acats_status_admin_event event to the admin-actions SSE stream, delivering real-time ACATS transfer status updates as a transfer moves through its lifecycle.

• A new acats_status_admin_event event type reports ACATS transfer status changes, carrying category: acats and created_by_kind: service.
• Adds flat belongs_to_kind, belongs_to_id_reference, created_by_kind, and created_by_id_reference fields to the event envelope.
• Deprecates the nested belongs_to and created_by objects, which will be removed on 2027-01-13; use the flat fields instead.
• Adds a discriminator on type so each event resolves to a single schema, and adds the note_admin_event type.
• Events are delivered only to correspondents with ACATS API access, the same status may be delivered more than once, and transfer_identifier is empty until the transfer is acknowledged.

Affected Endpoints

• GET /v2/events/admin-actions