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/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -78,6 +78,8 @@ export const IssueSchema = new Schema<IssueDoc>(
     outwardLinks: [link],
     inwardLinks: [link],
     watchers: [userRef],
+    parent: { _id: false, id: String, key: String, title: String },
+    checklist: [{ _id: false, id: String, text: String, done: Boolean }],
     attachments: [
       {
         _id: false,
@@ -99,6 +101,7 @@ IssueSchema.index({ projectId: 1, status: 1, updatedAt: -1 });
 IssueSchema.index({ projectId: 1, statusCategory: 1 });
 IssueSchema.index({ 'assignee.id': 1 });
 IssueSchema.index({ 'watchers.id': 1 });
+IssueSchema.index({ 'parent.id': 1 });
 IssueSchema.index({ 'release.id': 1 });
 IssueSchema.index({ title: 'text', description: 'text', key: 'text' });
 
@@ -182,5 +185,7 @@ export function issueFromDoc(doc: IssueDoc): IssueReadModel {
     // Tickets projected before attachments existed have none.
     attachments: rest.attachments ?? [],
     watchers: rest.watchers ?? [],
+    parent: rest.parent?.id ? rest.parent : null,
+    checklist: rest.checklist ?? [],
   };
 }