This is going well, I've added database level listener and at Type level (vertices, edges, documents).

Example of before-record-create listener where vertices with "validated" field equal to false cannot be saved (callback returns false):

database.getEvents().registerListener((BeforeRecordCreateListener) record -> record instanceof Vertex && record.asVertex().getBoolean("validated"));

The same by only for vertex type "Client":

database.getSchema().getType("Client").getEvents()
   .registerListener((BeforeRecordCreateListener) record -> record.asVertex().getBoolean("validated"));

Added new paragraph about events: https://docs.arcadedb.com/#Java-Events.