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/event-handlers.ts

@@ -28,6 +28,8 @@ import { IssueAttachmentAddedEventHandler } from './issue-attachment-added.event
 import { IssueAttachmentRemovedEventHandler } from './issue-attachment-removed.event-handler';
 import { AttachmentCleanupEventHandler } from './attachment-cleanup.event-handler';
 import { IssueWatchersEventHandler } from './issue-watchers.event-handler';
+import { IssueParentChangedEventHandler } from './issue-parent-changed.event-handler';
+import { IssueChecklistEventHandler } from './issue-checklist.event-handler';
 
 /** Read-model projections (MongoDB) + realtime, fed by the EventsBridge. */
 export const EVENT_HANDLERS = [
@@ -60,6 +62,8 @@ export const EVENT_HANDLERS = [
   IssueAttachmentRemovedEventHandler,
   AttachmentCleanupEventHandler,
   IssueWatchersEventHandler,
+  IssueParentChangedEventHandler,
+  IssueChecklistEventHandler,
 ];
 
 export { ReadModelNotifier };