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);
+  }
+}

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();
+  }
+}

libs/api/tracker/application/src/lib/queries/index.ts

@@ -17,3 +17,4 @@ export * from './search-issues.query';
 export * from './search-my-issues.query';
 export * from './get-public-projects.query';
 export * from './get-attachment.query';
+export * from './get-epics.query';

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,
 ];