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-handler.ts

@@ -0,0 +1,20 @@
+import { IQueryHandler, QueryHandler } from '@nestjs/cqrs';
+import { EpicReadModel } from '@task-stream/api/tracker/domain';
+import { FindIssuesRepository } from '../ports/find-issues.repository';
+import { GetEpicsQuery } from './get-epics.query';
+import { ReadAccess } from './read-access';
+
+@QueryHandler(GetEpicsQuery)
+export class GetEpicsQueryHandler
+  implements IQueryHandler<GetEpicsQuery, EpicReadModel[]>
+{
+  constructor(
+    private readonly findIssues: FindIssuesRepository,
+    private readonly access: ReadAccess,
+  ) {}
+
+  async execute(query: GetEpicsQuery): Promise<EpicReadModel[]> {
+    await this.access.assertCanView(query.projectId, query.viewerId);
+    return this.findIssues.epics(query.projectId);
+  }
+}