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/set-issue-parent.command.ts

@@ -0,0 +1,13 @@
+import { Command } from '@nestjs/cqrs';
+import { CommandResult } from './command-result';
+
+/** Places a ticket under an epic or another ticket (`parentId` null: out). */
+export class SetIssueParentCommand extends Command<CommandResult> {
+  constructor(
+    public readonly actorId: string,
+    public readonly issueId: string,
+    public readonly parentId: string | null,
+  ) {
+    super();
+  }
+}