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/eventhub_app.erl

@@ -118,6 +118,7 @@ start_http() ->
       {"/v1/events/:id/occurrences", handler_event_occurrences, []},
       {"/v1/events/:id/occurrences/:start_time", handler_event_occurrences, []},
       {"/v1/events/:id/bookings", handler_bookings, []},
+      {"/v1/events/:id/waitlist", handler_event_waitlist, []},
       {"/v1/bookings/:id", handler_booking_by_id, []},
       {"/v1/reviews", handler_reviews, []},
       {"/v1/reviews/:id", handler_review_by_id, []},