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/testing/in-memory.ts

@@ -256,6 +256,13 @@ export class InMemoryTrackerStreams extends TrackerStreamsRepository {
       (d) => (d['link'] as { targetIssueId: string }).targetIssueId === issueId,
     );
   }
+
+  async issueIdsEverUnder(issueId: string) {
+    return this.streams(
+      'IssueParentChangedEvent',
+      (d) => d['parentId'] === issueId,
+    );
+  }
 }
 
 /** Attachment storage double: files in a map, keyed like on disk. */