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/web/tracker/domain/src/lib/models/models.ts

@@ -40,6 +40,36 @@ export interface IssueLink {
   issueKey: string;
 }
 
+/** The ticket another one hangs from. */
+export interface IssueParent {
+  id: string;
+  key: string;
+  title: string;
+}
+
+/** A step inside a ticket that does not deserve a ticket of its own. */
+export interface ChecklistItem {
+  id: string;
+  text: string;
+  done: boolean;
+}
+
+/** How far the tickets hanging from one have got. */
+export interface ChildProgress {
+  total: number;
+  /** Done or cancelled. */
+  finished: number;
+  percent: number;
+  storyPoints: number;
+  finishedStoryPoints: number;
+}
+
+/** A top-level ticket that groups others. */
+export interface Epic {
+  issue: IssueSummary;
+  progress: ChildProgress;
+}
+
 /** What boards and lists show. */
 export interface IssueSummary {
   id: string;
@@ -55,6 +85,8 @@ export interface IssueSummary {
   assignee: UserRef | null;
   release: { id: string; name: string } | null;
   labels: string[];
+  /** The epic or ticket this one breaks down. */
+  parent: IssueParent | null;
   commentCount: number;
   updatedAt: string;
   version: number;
@@ -88,6 +120,7 @@ export interface Issue extends IssueSummary {
   attachments: Attachment[];
   /** People following the ticket. */
   watchers: UserRef[];
+  checklist: ChecklistItem[];
   allowedTransitions: string[];
   createdAt: string;
 }