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/web/tracker/domain/src/lib/models/models.ts

@@ -1,4 +1,5 @@
 import {
+  NotificationKind,
   IssueLinkType,
   IssuePriority,
   ProjectRole,
@@ -187,3 +188,25 @@ export interface CommandResult {
   key?: string | null;
   version: number;
 }
+
+/** Something the signed-in person should know. */
+export interface Notification {
+  id: string;
+  kind: NotificationKind;
+  projectId: string;
+  projectKey: string;
+  issueId: string | null;
+  issueKey: string | null;
+  actor: UserRef;
+  summary: string;
+  /** Null: unread. */
+  readAt: string | null;
+  createdAt: string;
+}
+
+export interface NotificationSettings {
+  /** Copies by e-mail. */
+  email: boolean;
+  /** Whether this installation can send e-mail at all. */
+  emailAvailable: boolean;
+}