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/sagas/sagas.spec.ts

@@ -39,12 +39,14 @@ import { ChangeIssueStatusCommand } from '../commands/change-issue-status.comman
 import { COMMAND_HANDLERS } from '../commands/command-handlers';
 import { CommandResult } from '../commands/command-result';
 import { CreateIssueCommand } from '../commands/create-issue.command';
+import { DeleteIssueCommand } from '../commands/delete-issue.command';
 import { CreateProjectCommand } from '../commands/create-project.command';
 import { CreateReleaseCommand } from '../commands/create-release.command';
 import { DeleteProjectCommand } from '../commands/delete-project.command';
 import { LinkIssuesCommand } from '../commands/link-issues.command';
 import { AssignIssueCommand } from '../commands/assign-issue.command';
 import { WriteAccess } from '../commands/write-access';
+import { IssueTree } from '../commands/issue-tree';
 import { FindIssuesRepository } from '../ports/find-issues.repository';
 import { IssueNumbersRepository } from '../ports/issue-numbers.repository';
 import { ProjectKeysRepository } from '../ports/project-keys.repository';
@@ -72,6 +74,7 @@ async function setup() {
       ReleaseArchivedSaga,
       UserDeactivatedSaga,
       WriteAccess,
+      IssueTree,
       AggregateRehydrator,
       { provide: EventStore, useValue: store },
       { provide: AttachmentStorage, useValue: files },
@@ -178,6 +181,31 @@ describe('sagas', () => {
     ).resolves.toMatchObject({ key: 'WEB' });
   });
 
+  it('a deleted ticket lets the tickets under it go back to the top', async () => {
+    const { run, drain, rehydrator } = await setup();
+    const web = (await run(new CreateProjectCommand('owner', 'WEB', 'Web'))).id;
+    const epic = (await run(ticket(web, 'Epic'))).id;
+    const child = (
+      await run(
+        new CreateIssueCommand(
+          'owner',
+          web,
+          'TASK',
+          'child',
+          {},
+          null,
+          null,
+          epic,
+        ),
+      )
+    ).id;
+
+    await run(new DeleteIssueCommand('owner', epic));
+    await drain();
+
+    expect((await rehydrator.rehydrate(child, Issue)).parentId).toBeNull();
+  });
+
   it('only the owner deletes a project', async () => {
     const { run } = await setup();
     const web = (await run(new CreateProjectCommand('owner', 'WEB', 'Web'))).id;