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>

apps/web/src/app/app.ts

@@ -11,6 +11,7 @@ import { filter, map } from 'rxjs';
 import { AuthFacade } from '@task-stream/web/identity/application';
 import { UserMenuComponent } from '@task-stream/web/identity/presenters';
 import { AppShellComponent } from '@task-stream/web/shared/ui';
+import { NotificationsBell } from '@task-stream/web/tracker/presenters';
 
 @Component({
   selector: 'ts-root',
@@ -21,6 +22,7 @@ import { AppShellComponent } from '@task-stream/web/shared/ui';
     RouterLinkActive,
     AppShellComponent,
     UserMenuComponent,
+    NotificationsBell,
   ],
   template: `
     <ts-app-shell>
@@ -44,6 +46,7 @@ import { AppShellComponent } from '@task-stream/web/shared/ui';
         Explorar
       </a>
       @if (auth.isAuthenticated()) {
+        <ts-notifications-bell shellUser />
         <ts-user-menu shellUser />
       } @else if (auth.settled() && !onLogin()) {
         <a shellUser class="ts-nav-link ts-nav-link--cta" routerLink="/login"