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/ui/issue-card.ts

@@ -40,6 +40,11 @@ import { initials, PRIORITY, PRIORITY_LEVEL, typeLook } from './copy';
         >
           <i></i><i></i><i></i><i></i>
         </span>
+        @if (issue().parent; as parent) {
+          <span class="ts-card__meta" [attr.title]="'Cuelga de ' + parent.title"
+            >↑ {{ parent.key }}</span
+          >
+        }
         @if (issue().release; as release) {
           <span class="ts-card__meta" title="Release"
             >◆ {{ release.name }}</span