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/logic/logic_email.erl

@@ -1,6 +1,6 @@
 -module(logic_email).
 -export([send_verification_email/2, send_specialist_invite/2, send_password_reset/2,
-         send_booking_reminder/4]).
+         send_booking_reminder/4, send_waitlist_promoted/3]).
 
 %% SMTP via env (variant A / Back#68):
 %%   SMTP_HOST empty → log-only (local/tests)
@@ -29,6 +29,13 @@ send_booking_reminder(Email, EventTitle, StartTime, EventPath) ->
     logic_email_templates:render_reminder(Url, to_bin(EventTitle), WhenText),
   send(to_bin(Email), Subject, Plain, Html).
 
+-spec send_waitlist_promoted(binary(), binary(), binary()) -> ok.
+send_waitlist_promoted(Email, EventTitle, EventPath) ->
+  Url = abs_path(EventPath),
+  {Subject, Plain, Html} =
+    logic_email_templates:render_waitlist_promoted(Url, to_bin(EventTitle)),
+  send(to_bin(Email), Subject, Plain, Html).
+
 send_kind(Kind, To0, Url) ->
   {Subject, Plain, Html} = logic_email_templates:render(Kind, Url),
   send(to_bin(To0), Subject, Plain, Html).