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/infrastructure/src/lib/tracker-infrastructure.providers.ts

@@ -7,6 +7,7 @@ import {
   AttachmentsGateway,
   CommentsGateway,
   IssuesGateway,
+  NotificationsGateway,
   ProjectsGateway,
   ReleasesGateway,
   TrackerRealtimeGateway,
@@ -19,6 +20,7 @@ import {
   GraphqlUsersGateway,
 } from './adapters/graphql-collaboration.gateways';
 import { GraphqlIssuesGateway } from './adapters/graphql-issues.gateway';
+import { GraphqlNotificationsGateway } from './adapters/graphql-notifications.gateway';
 import { HttpAttachmentsGateway } from './adapters/http-attachments.gateway';
 import { GraphqlProjectsGateway } from './adapters/graphql-projects.gateway';
 import { GraphqlRealtimeGateway } from './adapters/graphql-realtime.gateway';
@@ -33,6 +35,7 @@ export function provideTrackerInfrastructure(): EnvironmentProviders {
     { provide: ActivityGateway, useClass: GraphqlActivityGateway },
     { provide: AttachmentsGateway, useClass: HttpAttachmentsGateway },
     { provide: UsersGateway, useClass: GraphqlUsersGateway },
+    { provide: NotificationsGateway, useClass: GraphqlNotificationsGateway },
     { provide: TrackerRealtimeGateway, useClass: GraphqlRealtimeGateway },
   ]);
 }