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/requests.dto.ts

@@ -408,6 +408,14 @@ export class IssueSearchQueryDto extends PageQueryDto {
   @IsString({ each: true })
   label?: string[];
 
+  @ApiPropertyOptional({
+    description: 'Only the tickets hanging from this one (id or key)',
+    example: 'WEB-3',
+  })
+  @IsOptional()
+  @IsString()
+  parent?: string;
+
   @ApiPropertyOptional({ description: 'Matches key, title or description' })
   @IsOptional()
   @IsString()
@@ -501,6 +509,17 @@ export class CreateIssueDto extends IssueFieldsDto {
   @IsOptional()
   @IsString()
   releaseId?: string | null;
+
+  @ApiPropertyOptional({
+    type: String,
+    nullable: true,
+    example: 'WEB-3',
+    description:
+      'Epic or ticket of the same project the new one breaks down (id or key)',
+  })
+  @IsOptional()
+  @IsString()
+  parent?: string | null;
 }
 
 export class UpdateIssueDto extends IssueFieldsDto {
@@ -560,6 +579,41 @@ export class SetIssueReleaseDto {
   releaseId!: string | null;
 }
 
+export class SetIssueParentDto {
+  @ApiProperty({
+    type: String,
+    nullable: true,
+    example: 'WEB-3',
+    description:
+      'Epic or ticket of the same project (id or key), or null to take it back to the top',
+  })
+  @ValidateIf((dto: SetIssueParentDto) => dto.parent !== null)
+  @IsString()
+  parent!: string | null;
+}
+
+export class ChecklistItemBodyDto {
+  @ApiProperty({ example: 'Actualizar el changelog', maxLength: 300 })
+  @IsString()
+  @MinLength(1)
+  @MaxLength(300)
+  text!: string;
+}
+
+export class UpdateChecklistItemDto {
+  @ApiPropertyOptional({ maxLength: 300 })
+  @IsOptional()
+  @IsString()
+  @MinLength(1)
+  @MaxLength(300)
+  text?: string;
+
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsBoolean()
+  done?: boolean;
+}
+
 export class LinkIssueDto {
   @ApiProperty({ enum: IssueLinkType, enumName: 'IssueLinkType' })
   @IsEnum(IssueLinkType)