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/index.ts

@@ -6,3 +6,4 @@ export * from './lib/facades/issue.facade';
 export * from './lib/facades/home.facade';
 export * from './lib/facades/explore.facade';
 export * from './lib/facades/inbox.facade';
+export * from './lib/facades/epics.facade';