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/commands.spec.ts

@@ -28,7 +28,15 @@ import {
   InMemoryProjectKeys,
   InMemoryReleaseNames,
   InMemoryReleaseIssues,
+  InMemoryTrackerStreams,
 } from '../../testing/in-memory';
+import { TrackerStreamsRepository } from '../ports/tracker-streams.repository';
+import { IssueTree } from './issue-tree';
+import { SetIssueParentCommand } from './set-issue-parent.command';
+import {
+  AddChecklistItemCommand,
+  UpdateChecklistItemCommand,
+} from './change-checklist.command';
 import { ReleaseIssuesRepository } from '../ports/release-issues.repository';
 import { IssueNumbersRepository } from '../ports/issue-numbers.repository';
 import { ProjectKeysRepository } from '../ports/project-keys.repository';
@@ -67,6 +75,7 @@ async function setup() {
       ...COMMAND_HANDLERS,
       ...TRACKER_FACTORIES,
       WriteAccess,
+      IssueTree,
       AggregateRehydrator,
       { provide: EventStore, useValue: store },
       { provide: AttachmentStorage, useValue: files },
@@ -78,6 +87,10 @@ async function setup() {
         provide: ReleaseIssuesRepository,
         useValue: new InMemoryReleaseIssues(store),
       },
+      {
+        provide: TrackerStreamsRepository,
+        useValue: new InMemoryTrackerStreams(store),
+      },
     ],
   }).compile();
   await moduleRef.init();
@@ -281,6 +294,84 @@ describe('tracker commands (event sourced)', () => {
     ).rejects.toThrow(ForbiddenError);
   });
 
+  describe('epics, subtasks and checklists', () => {
+    const create = (
+      run: (c: object) => Promise<CommandResult>,
+      projectId: string,
+      title: string,
+      parentId?: string,
+    ) =>
+      run(
+        new CreateIssueCommand(
+          'dev',
+          projectId,
+          IssueType.TASK,
+          title,
+          {},
+          null,
+          null,
+          parentId,
+        ),
+      );
+
+    it('nests tickets three levels deep at most, without loops', async () => {
+      const { run, rehydrator } = await setup();
+      const projectId = await projectWithTeam(run);
+      const epic = await create(run, projectId, 'Checkout');
+      const story = await create(run, projectId, 'Pay by card', epic.id);
+      const subtask = await create(run, projectId, 'Stripe form', story.id);
+      const loose = await create(run, projectId, 'Loose ticket');
+
+      expect((await rehydrator.rehydrate(subtask.id, Issue)).parentId).toBe(
+        story.id,
+      );
+      // A fourth level, and a loop.
+      await expect(
+        create(run, projectId, 'Too deep', subtask.id),
+      ).rejects.toThrow(/at most 2 levels/);
+      await expect(
+        run(new SetIssueParentCommand('dev', epic.id, subtask.id)),
+      ).rejects.toThrow(BusinessRuleError);
+      // A ticket with subtasks cannot go under one that has a parent.
+      await run(new SetIssueParentCommand('dev', loose.id, epic.id));
+      await expect(
+        run(new SetIssueParentCommand('dev', story.id, loose.id)),
+      ).rejects.toThrow(/at most 2 levels/);
+      // ...but it can move to another epic, and leave.
+      await run(new SetIssueParentCommand('dev', loose.id, null));
+      await run(new SetIssueParentCommand('dev', story.id, loose.id));
+      expect((await rehydrator.rehydrate(story.id, Issue)).parentId).toBe(
+        loose.id,
+      );
+      await expect(
+        run(new SetIssueParentCommand('pm', story.id, null)),
+      ).rejects.toThrow(ForbiddenError);
+    });
+
+    it('keeps a checklist on the ticket', async () => {
+      const { run, rehydrator } = await setup();
+      const projectId = await projectWithTeam(run);
+      const issue = await create(run, projectId, 'Release 1.0');
+      await run(new AddChecklistItemCommand('dev', issue.id, 'Changelog'));
+      await run(new AddChecklistItemCommand('qa', issue.id, 'Smoke test'));
+      const [first] = (await rehydrator.rehydrate(issue.id, Issue)).checklist;
+      await run(
+        new UpdateChecklistItemCommand('qa', issue.id, first.id, {
+          done: true,
+        }),
+      );
+
+      const { checklist } = await rehydrator.rehydrate(issue.id, Issue);
+      expect(checklist.map((i) => [i.text, i.done])).toEqual([
+        ['Changelog', true],
+        ['Smoke test', false],
+      ]);
+      await expect(
+        run(new AddChecklistItemCommand('pm', issue.id, 'Nope')),
+      ).rejects.toThrow(ForbiddenError);
+    });
+  });
+
   it('does not let viewers modify tickets', async () => {
     const { run } = await setup();
     const projectId = await projectWithTeam(run);