feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/notifications/notifications.spec.ts

@@ -54,13 +54,21 @@ function setup(mailEnabled = false) {
   const repo = new InMemoryNotifications();
   const published: NotificationReadModel[] = [];
   const mailed: string[] = [];
-  const users = new FakeUserDirectory().add('ana').add('bob').add('qa');
+  const users = new FakeUserDirectory()
+    .add('ana')
+    .add('bob')
+    .add('qa')
+    .add('pm');
   const issue = {
     id: 'i-1',
     key: 'WEB-1',
     title: 'Login roto',
     reporter: { id: 'qa', name: 'QA' },
     assignee: { id: 'bob', name: 'Bob' },
+    watchers: [
+      { id: 'pm', name: 'PM' },
+      { id: 'bob', name: 'Bob' },
+    ],
   };
   const notifier = new Notifier(
     repo,
@@ -81,7 +89,15 @@ function setup(mailEnabled = false) {
     notifier,
     { findById: async () => issue } as unknown as FindIssuesRepository,
     {
-      findById: async () => ({ id: 'p-1', key: 'WEB', name: 'Web' }),
+      findById: async () => ({
+        id: 'p-1',
+        key: 'WEB',
+        name: 'Web',
+        members: ['ana', 'bob', 'qa', 'pm'].map((id) => ({
+          user: { id, name: id },
+          role: ProjectRole.MEMBER,
+        })),
+      }),
     } as unknown as FindProjectsRepository,
     users,
   );
@@ -100,8 +116,12 @@ describe('notifications', () => {
 
     expect(repo.all.map((n) => [n.userId, n.kind])).toEqual([
       ['bob', NotificationKind.ASSIGNED],
+      // Bob follows the ticket too, but is told once: as the assignee.
+      ['pm', NotificationKind.WATCHED],
+      ['pm', NotificationKind.WATCHED],
+      ['bob', NotificationKind.WATCHED],
     ]);
-    expect(published).toHaveLength(1);
+    expect(published).toHaveLength(4);
     expect(repo.all[0].summary).toContain('WEB-1');
   });
 
@@ -129,6 +149,7 @@ describe('notifications', () => {
     expect(repo.all.map((n) => [n.userId, n.kind])).toEqual([
       ['bob', NotificationKind.QA_FAILED],
       ['qa', NotificationKind.QA_FAILED],
+      ['pm', NotificationKind.WATCHED],
     ]);
     expect(repo.all[0].summary).toContain('Falla en Safari');
   });
@@ -151,7 +172,32 @@ describe('notifications', () => {
     await handler.handle(comment);
     await handler.handle(comment);
 
-    expect(repo.all.map((n) => n.userId)).toEqual(['qa']);
+    expect(repo.all.map((n) => n.userId)).toEqual(['qa', 'pm']);
+  });
+
+  it('tells a mentioned member first, and nobody outside the project', async () => {
+    const { handler, repo } = setup();
+    await handler.handle(
+      at(
+        new CommentAddedEvent('bob', {
+          id: 'c-2',
+          issueId: 'i-1',
+          projectId: 'p-1',
+          authorId: 'bob',
+          body: '@[PM](pm) mira esto, y @[Fuera](stranger) también',
+          deleted: false,
+          createdAt: '2026-10-08T10:00:00.000Z',
+          editedAt: null,
+        }),
+        1,
+      ),
+    );
+
+    expect(repo.all.map((n) => [n.userId, n.kind])).toEqual([
+      ['pm', NotificationKind.MENTIONED],
+      ['qa', NotificationKind.COMMENTED],
+    ]);
+    expect(repo.all[0].summary).toContain('«@PM mira esto');
   });
 
   it('copies by e-mail only to whoever asked for it, when mail is configured', async () => {