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/domain/src/lib/read-models/issue.read-model.ts

@@ -47,6 +47,8 @@ export class IssueReadModel {
   dueDate!: string | null;
   links!: IssueLinkReadModel[];
   attachments!: AttachmentReadModel[];
+  /** People following the ticket. */
+  watchers!: UserRefReadModel[];
   commentCount!: number;
   createdAt!: string;
   updatedAt!: string;