feat(notify): Web Push subscriptions and email/push prefs.

Prefs in user.preferences; push_subscription table; VAPID send on
reminder/waitlist. Refs EventHub/EventHubBack#75

include/records.hrl

@@ -336,6 +336,17 @@
   created_at      :: calendar:datetime()
 }).
 
+%% Web Push subscription (Back#75). Prefs live in user.preferences:
+%% notify_email / notify_push (boolean, default true).
+-record(push_subscription, {
+  id              :: binary(),
+  user_id         :: binary(),
+  endpoint        :: binary(),
+  p256dh          :: binary(),
+  auth            :: binary(),
+  created_at      :: calendar:datetime()
+}).
+
 %% Upsert-счётчики админ-статистики (абсолютные значения).
 %% key: atom() | {atom(), atom()} | {atom(), atom(), atom()}
 %%   users_total-подобные totals не храним (table_info),