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/lib/ports/index.ts

@@ -18,3 +18,5 @@ export * from './upsert-materialized-timeline.repository';
 export * from './find-timeline.repository';
 export * from './tracker-streams.repository';
 export * from './attachment-storage';
+export * from './notifications.repository';
+export * from './notification-mailer';