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/migrations/20260814193000_waitlist_entry.erl

@@ -0,0 +1,38 @@
+%% @doc Create waitlist_entry table and indexes (Back#72).
+-module('20260814193000_waitlist_entry').
+
+-export([up/0, down/0]).
+
+-include("records.hrl").
+
+up() ->
+  ensure_table(waitlist_entry, record_info(fields, waitlist_entry)),
+  ensure_index(waitlist_entry, event_id),
+  ensure_index(waitlist_entry, user_id),
+  ensure_index(waitlist_entry, status),
+  ok.
+
+down() ->
+  _ = mnesia:delete_table(waitlist_entry),
+  ok.
+
+ensure_table(Table, Attrs) ->
+  case lists:member(Table, mnesia:system_info(tables)) of
+    true ->
+      ok;
+    false ->
+      case mnesia:create_table(Table, [{disc_copies, [node()]}, {attributes, Attrs}]) of
+        {atomic, ok} -> ok;
+        {aborted, {already_exists, Table}} -> ok;
+        {aborted, Reason} -> error({create_table_failed, Table, Reason})
+      end
+  end.
+
+ensure_index(Table, Attr) ->
+  case mnesia:add_table_index(Table, Attr) of
+    {atomic, ok} -> ok;
+    {aborted, {already_exists, Table, _Pos}} -> ok;
+    {aborted, {already_exists, Table, Attr}} -> ok;
+    {aborted, {already_exists, _}} -> ok;
+    {aborted, Reason} -> error({add_index_failed, Table, Attr, Reason})
+  end.