Adds a reaction of the caller to a reply and publishes an EAppMailReactedEvent carrying the reply. Only one reaction per reaction value per user. Returns the updated record.

The record is looked up within the caller's tenant, and the caller must also be allowed to access the entity the email is linked to (employer, customer, lead, workflow or meeting, checked in that order); otherwise that entity's access check rejects the request. The caller also needs the update permission of the linked entity type (employers:write, leads:update, workflows:update, customers:update or meetings:update), checked in the service layer.

Errors: 400 if the caller already has this reaction on the reply; 403 if the update permission is missing; 404 if the record or the reply does not exist.