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/web/tracker/domain/src/lib/models/models.ts

@@ -86,6 +86,8 @@ export interface Issue extends IssueSummary {
   dueDate: string | null;
   links: IssueLink[];
   attachments: Attachment[];
+  /** People following the ticket. */
+  watchers: UserRef[];
   allowedTransitions: string[];
   createdAt: string;
 }