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/notification-mailer.ts

@@ -0,0 +1,16 @@
+import { NotificationReadModel } from '@task-stream/api/tracker/domain';
+
+export interface MailRecipient {
+  name: string;
+  email: string;
+}
+
+/** E-mail copies of notifications, for whoever asked for them. */
+export abstract class NotificationMailer {
+  /** False when no mail server is configured: nobody is offered e-mail. */
+  abstract get enabled(): boolean;
+  abstract send(
+    to: MailRecipient,
+    notification: NotificationReadModel,
+  ): Promise<void>;
+}