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/tracker-application.module.ts

@@ -4,6 +4,7 @@ import { TRACKER_FACTORIES } from '@task-stream/api/tracker/domain';
 import { ChangesService } from './changes.service';
 import { COMMAND_HANDLERS } from './commands/command-handlers';
 import { WriteAccess } from './commands/write-access';
+import { IssueTree } from './commands/issue-tree';
 import { CommentsService } from './comments.service';
 import {
   EVENT_HANDLERS,
@@ -59,6 +60,7 @@ const SERVICES = [
     ...SERVICES,
     ...TRACKER_FACTORIES,
     WriteAccess,
+    IssueTree,
     ReadAccess,
     ReadModelNotifier,
     ...COMMAND_HANDLERS,