feat: notifications in the app and, optionally, by e-mail

Derived from events and kept per person: whoever a ticket is assigned to,
whoever reports or works on it when it moves, fails QA or gets a comment,
and whoever joins a project. Never about what you did yourself, once per
event. They arrive live through a subscription to a bell in the header,
and are copied by e-mail to whoever turns it on when SMTP_URL is set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/presenters/src/lib/graphql/enums.ts

@@ -2,6 +2,7 @@ import { registerEnumType } from '@nestjs/graphql';
 import {
   IssueLinkType,
   IssuePriority,
+  NotificationKind,
   ProjectRole,
   ReleaseStatus,
   StatusCategory,
@@ -22,3 +23,7 @@ registerEnumType(IssueLinkType, { name: 'IssueLinkType' });
 registerEnumType(ProjectRole, { name: 'ProjectRole' });
 registerEnumType(ReleaseStatus, { name: 'ReleaseStatus' });
 registerEnumType(LinkDirection, { name: 'LinkDirection' });
+registerEnumType(NotificationKind, {
+  name: 'NotificationKind',
+  description: 'Why someone is told',
+});