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/web/tracker/presenters/src/lib/tracker.routes.ts

@@ -38,6 +38,12 @@ export const trackerRoutes: Routes = [
         loadComponent: () =>
           import('./pages/issue.page').then((m) => m.IssuePage),
       },
+      {
+        path: 'epics',
+        title: 'Épicas · TaskStream',
+        loadComponent: () =>
+          import('./pages/epics.page').then((m) => m.EpicsPage),
+      },
       {
         path: 'releases',
         title: 'Releases · TaskStream',