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/activity.event-handler.ts

@@ -7,6 +7,10 @@ import {
   IssueAssignedEvent,
   IssueAttachmentAddedEvent,
   IssueAttachmentRemovedEvent,
+  IssueChecklistItemAddedEvent,
+  IssueChecklistItemRemovedEvent,
+  IssueChecklistItemUpdatedEvent,
+  IssueParentChangedEvent,
   IssueCreatedEvent,
   IssueLinkedEvent,
   IssueReleaseChangedEvent,
@@ -55,6 +59,10 @@ import { versionOf } from './read-model-notifier';
   IssueReleaseChangedEvent,
   IssueAttachmentAddedEvent,
   IssueAttachmentRemovedEvent,
+  IssueParentChangedEvent,
+  IssueChecklistItemAddedEvent,
+  IssueChecklistItemUpdatedEvent,
+  IssueChecklistItemRemovedEvent,
   CommentAddedEvent,
   CommentEditedEvent,
   CommentDeletedEvent,