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/presenters/src/lib/graphql/subscription.resolver.ts

@@ -1,11 +1,15 @@
 import { Args, ID, Resolver, Subscription } from '@nestjs/graphql';
 import { User } from '@task-stream/api/identity/domain';
 import { CurrentUser } from '@task-stream/api/identity/presenters';
-import { ChangesService } from '@task-stream/api/tracker/application';
+import {
+  ChangesService,
+  NotificationsService,
+} from '@task-stream/api/tracker/application';
 import {
   ActivityGql,
   CommentChangeGql,
   IssueChangeGql,
+  NotificationGql,
   ProjectChangeGql,
   ReleaseChangeGql,
 } from './types';
@@ -18,7 +22,10 @@ const asIs = { resolve: <T>(payload: T) => payload };
  */
 @Resolver()
 export class SubscriptionResolver {
-  constructor(private readonly changes: ChangesService) {}
+  constructor(
+    private readonly changes: ChangesService,
+    private readonly notifications: NotificationsService,
+  ) {}
 
   @Subscription(() => IssueChangeGql, {
     ...asIs,
@@ -74,4 +81,12 @@ export class SubscriptionResolver {
   myProjectsChanged(@CurrentUser() me: User) {
     return this.changes.watchMyProjects(me.id);
   }
+
+  @Subscription(() => NotificationGql, {
+    ...asIs,
+    description: 'My notifications, as they happen',
+  })
+  notificationAdded(@CurrentUser() me: User) {
+    return this.notifications.watch(me.id);
+  }
 }