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/domain/src/lib/read-models/issue.read-model.ts

@@ -23,6 +23,35 @@ export class AttachmentReadModel {
   commentId!: string | null;
 }
 
+export class ChecklistItemReadModel {
+  id!: string;
+  text!: string;
+  done!: boolean;
+}
+
+/** The ticket another one hangs from. */
+export class IssueParentReadModel {
+  id!: string;
+  key!: string;
+  title!: string;
+}
+
+/** How far the tickets hanging from one have got. */
+export class ChildProgressReadModel {
+  total!: number;
+  /** Done or cancelled. */
+  finished!: number;
+  percent!: number;
+  storyPoints!: number;
+  finishedStoryPoints!: number;
+}
+
+/** A top-level ticket that groups others, with their progress. */
+export class EpicReadModel {
+  issue!: IssueReadModel;
+  progress!: ChildProgressReadModel;
+}
+
 export class IssueReadModel {
   id!: string;
   projectId!: string;
@@ -49,6 +78,8 @@ export class IssueReadModel {
   attachments!: AttachmentReadModel[];
   /** People following the ticket. */
   watchers!: UserRefReadModel[];
+  parent!: IssueParentReadModel | null;
+  checklist!: ChecklistItemReadModel[];
   commentCount!: number;
   createdAt!: string;
   updatedAt!: string;