feat(waitlist): optional event waitlist with FIFO promote.

Commercial calendars opt in via settings.waitlist_enabled; join when full;
auto-promote on cancel/decline/expire with email and in-app notify.

Refs EventHub/EventHubBack#72

src/swagger/eventhub_trails.erl

@@ -87,6 +87,7 @@ user() ->
     handler_event_by_id,
     handler_event_occurrences,
     handler_events,
+    handler_event_waitlist,
     handler_reports,
     handler_review_by_id,
     handler_review_vote,