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

test/unit/eh_test_support.erl

@@ -87,6 +87,7 @@ ensure_indexes(Tables) when is_list(Tables) ->
     {event, [calendar_id, title, created_at, start_time, event_type,
              master_id, specialist_id, status]},
     {booking, [event_id, user_id, status]},
+    {waitlist_entry, [event_id, user_id, status]},
     {review_vote, [review_id, user_id]},
     {calendar, [owner_id, status, short_name, category]},
     {calendar_specialist, [calendar_id, user_id]},
@@ -156,6 +157,8 @@ table_opts(recurrence_exception) ->
   [{ram_copies, [node()]}, {type, bag}, {attributes, record_info(fields, recurrence_exception)}];
 table_opts(booking) ->
   [{ram_copies, [node()]}, {attributes, record_info(fields, booking)}];
+table_opts(waitlist_entry) ->
+  [{ram_copies, [node()]}, {attributes, record_info(fields, waitlist_entry)}];
 table_opts(review) ->
   [{ram_copies, [node()]}, {attributes, record_info(fields, review)}];
 table_opts(review_vote) ->