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/commands/command-handlers.ts

@@ -28,6 +28,13 @@ import { UnassignIssueCommandHandler } from './unassign-issue.command-handler';
 import { AttachFileCommandHandler } from './attach-file.command-handler';
 import { DetachFileCommandHandler } from './detach-file.command-handler';
 import { WatchIssueCommandHandler } from './watch-issue.command-handler';
+import { SetIssueParentCommandHandler } from './set-issue-parent.command-handler';
+import { DetachFromParentCommandHandler } from './detach-from-parent.command-handler';
+import {
+  AddChecklistItemCommandHandler,
+  RemoveChecklistItemCommandHandler,
+  UpdateChecklistItemCommandHandler,
+} from './change-checklist.command-handler';
 
 export const COMMAND_HANDLERS = [
   CreateProjectCommandHandler,
@@ -61,4 +68,10 @@ export const COMMAND_HANDLERS = [
   AttachFileCommandHandler,
   DetachFileCommandHandler,
   WatchIssueCommandHandler,
+  SetIssueParentCommandHandler,
+  AddChecklistItemCommandHandler,
+  UpdateChecklistItemCommandHandler,
+  RemoveChecklistItemCommandHandler,
+  // Saga only: the parent was deleted.
+  DetachFromParentCommandHandler,
 ];