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/queries/query-handlers.ts

@@ -16,6 +16,7 @@ import { GetReleaseProgressQueryHandler } from './get-release-progress.query-han
 import { GetReleasesQueryHandler } from './get-releases.query-handler';
 import { GetUpcomingReleasesQueryHandler } from './get-upcoming-releases.query-handler';
 import { SearchIssuesQueryHandler } from './search-issues.query-handler';
+import { GetEpicsQueryHandler } from './get-epics.query-handler';
 import { SearchMyIssuesQueryHandler } from './search-my-issues.query-handler';
 
 export const QUERY_HANDLERS = [
@@ -36,6 +37,7 @@ export const QUERY_HANDLERS = [
   GetReleasesQueryHandler,
   GetUpcomingReleasesQueryHandler,
   SearchIssuesQueryHandler,
+  GetEpicsQueryHandler,
   SearchMyIssuesQueryHandler,
   GetAttachmentQueryHandler,
 ];