feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

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

libs/api/tracker/presenters/src/lib/http/dto/responses.dto.ts

@@ -123,6 +123,18 @@ export class AttachmentDto {
   commentId!: string | null;
 }
 
+export class ChecklistItemDto {
+  @ApiProperty({ format: 'uuid' }) id!: string;
+  @ApiProperty({ example: 'Actualizar el changelog' }) text!: string;
+  @ApiProperty() done!: boolean;
+}
+
+export class IssueParentDto {
+  @ApiProperty({ format: 'uuid' }) id!: string;
+  @ApiProperty({ example: 'WEB-3' }) key!: string;
+  @ApiProperty({ example: 'Pago con tarjeta' }) title!: string;
+}
+
 export class IssueDto {
   @ApiProperty({ format: 'uuid' }) id!: string;
   @ApiProperty({ format: 'uuid' }) projectId!: string;
@@ -161,6 +173,13 @@ export class IssueDto {
   @ApiProperty({ type: [AttachmentDto] }) attachments!: AttachmentDto[];
   @ApiProperty({ type: [UserRefDto], description: 'People following it' })
   watchers!: UserRefDto[];
+  @ApiProperty({
+    type: IssueParentDto,
+    nullable: true,
+    description: 'The epic or ticket this one breaks down',
+  })
+  parent!: IssueParentDto | null;
+  @ApiProperty({ type: [ChecklistItemDto] }) checklist!: ChecklistItemDto[];
   @ApiProperty() commentCount!: number;
   @ApiProperty({ format: 'date-time' }) createdAt!: string;
   @ApiProperty({ format: 'date-time' }) updatedAt!: string;
@@ -233,6 +252,19 @@ export class CountByAssigneeDto {
   @ApiProperty() count!: number;
 }
 
+export class ChildProgressDto {
+  @ApiProperty() total!: number;
+  @ApiProperty({ description: 'Done or cancelled' }) finished!: number;
+  @ApiProperty({ minimum: 0, maximum: 100 }) percent!: number;
+  @ApiProperty() storyPoints!: number;
+  @ApiProperty() finishedStoryPoints!: number;
+}
+
+export class EpicDto {
+  @ApiProperty({ type: IssueDto }) issue!: IssueDto;
+  @ApiProperty({ type: ChildProgressDto }) progress!: ChildProgressDto;
+}
+
 export class ReleaseProgressDto {
   @ApiProperty() total!: number;
   @ApiProperty() finished!: number;