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/lib/event-handlers/activity-summary.ts

@@ -5,6 +5,10 @@ import {
   IssueAssignedEvent,
   IssueAttachmentAddedEvent,
   IssueAttachmentRemovedEvent,
+  IssueChecklistItemAddedEvent,
+  IssueChecklistItemRemovedEvent,
+  IssueChecklistItemUpdatedEvent,
+  IssueParentChangedEvent,
   IssueCreatedEvent,
   IssueLinkedEvent,
   IssueLinkType,
@@ -142,6 +146,23 @@ export function describeEvent(
   if (event instanceof IssueAttachmentRemovedEvent)
     return `${actor} quitó el adjunto ${event.fileName} de ${issueKey}`;
 
+  if (event instanceof IssueParentChangedEvent) {
+    return event.parentKey
+      ? `${actor} colocó ${issueKey} bajo ${event.parentKey}`
+      : `${actor} sacó ${issueKey} de su ticket padre`;
+  }
+  if (event instanceof IssueChecklistItemAddedEvent)
+    return `${actor} añadió «${event.item.text}» a la lista de ${issueKey}`;
+  if (event instanceof IssueChecklistItemUpdatedEvent) {
+    if (event.changes.done === true)
+      return `${actor} completó un paso de la lista de ${issueKey}`;
+    if (event.changes.done === false)
+      return `${actor} reabrió un paso de la lista de ${issueKey}`;
+    return `${actor} cambió un paso de la lista de ${issueKey}`;
+  }
+  if (event instanceof IssueChecklistItemRemovedEvent)
+    return `${actor} quitó «${event.text}» de la lista de ${issueKey}`;
+
   if (event instanceof CommentAddedEvent)
     return `${actor} comentó en ${issueKey}`;
   if (event instanceof CommentEditedEvent)