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/application/src/index.ts

@@ -7,4 +7,6 @@ export * from './lib/comments.service';
 export * from './lib/releases.service';
 export * from './lib/changes.service';
 export * from './lib/workflows.service';
+export * from './lib/notifications.service';
+export * from './lib/notifications';
 export * from './lib/tracker-application.module';