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/application/src/lib/event-handlers/issue-created.event-handler.ts

@@ -48,6 +48,7 @@ export class IssueCreatedEventHandler
       reporter: users.of(issue.reporterId),
       assignee: users.ofNullable(issue.assigneeId),
       release: release ? { id: release.id, name: release.name } : null,
+      parent: null,
       labels: issue.labels,
       figmaUrl: issue.figmaUrl,
       storyPoints: issue.storyPoints,