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/presenters/src/lib/http/dto/responses.dto.ts

@@ -159,6 +159,8 @@ export class IssueDto {
   dueDate!: string | null;
   @ApiProperty({ type: [IssueLinkDto] }) links!: IssueLinkDto[];
   @ApiProperty({ type: [AttachmentDto] }) attachments!: AttachmentDto[];
+  @ApiProperty({ type: [UserRefDto], description: 'People following it' })
+  watchers!: UserRefDto[];
   @ApiProperty() commentCount!: number;
   @ApiProperty({ format: 'date-time' }) createdAt!: string;
   @ApiProperty({ format: 'date-time' }) updatedAt!: string;