backend, psql: implemented notifications for auto subscribers in PEP: people with presence permission from node owner and who request notification (+notify) from node will receive notifications according to XEP-0163 §4. This make SàT Pubsub a nearly fully compliant PEP service. Following features are still missing: - presence access model, should be implemented very soon as everything is already there - deleting resource on unavailable presence, will be implemented really soon - XEP-0115 hash check - and most importantly, rosters updates. XEP-0356 needs to be updated for this.

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);