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/get-epics.query.ts

@@ -0,0 +1,13 @@
+import { Query } from '@nestjs/cqrs';
+import { EpicReadModel } from '@task-stream/api/tracker/domain';
+
+/** A project's epics: top-level tickets that group others, with progress. */
+export class GetEpicsQuery extends Query<EpicReadModel[]> {
+  constructor(
+    /** Null: anonymous (public projects only). */
+    public readonly viewerId: string | null,
+    public readonly projectId: string,
+  ) {
+    super();
+  }
+}