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/application/src/lib/events/epics.events.ts

@@ -0,0 +1,18 @@
+import { type } from '@ngrx/signals';
+import { eventGroup } from '@ngrx/signals/events';
+import { Epic } from '@task-stream/web/tracker/domain';
+
+export const epicsPageEvents = eventGroup({
+  source: 'Epics Page',
+  events: {
+    opened: type<{ projectId: string }>(),
+  },
+});
+
+export const epicsApiEvents = eventGroup({
+  source: 'Epics API',
+  events: {
+    loaded: type<Epic[]>(),
+    loadFailed: type<string>(),
+  },
+});