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/ports/gateways.ts

@@ -87,6 +87,8 @@ export abstract class IssuesGateway {
     issueId: string,
     releaseId: string | null,
   ): Observable<CommandResult>;
+  /** The signed-in person follows (or stops following) the ticket. */
+  abstract watch(issueId: string, watching: boolean): Observable<CommandResult>;
 }
 
 export abstract class CommentsGateway {