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/upsert-materialized.repositories.ts

@@ -11,6 +11,7 @@ import {
 import {
   ActivityReadModel,
   AttachmentReadModel,
+  ChecklistItemReadModel,
   CommentReadModel,
   IssueLinkReadModel,
   IssueReadModel,
@@ -130,6 +131,45 @@ export class MongoUpsertMaterializedIssueRepository extends UpsertMaterializedIs
     );
   }
 
+  async addChecklistItem(issueId: string, item: ChecklistItemReadModel) {
+    await this.issues.updateOne(
+      { _id: issueId, 'checklist.id': { $ne: item.id } },
+      { $push: { checklist: item } },
+    );
+  }
+
+  async updateChecklistItem(
+    issueId: string,
+    itemId: string,
+    changes: Partial<Pick<ChecklistItemReadModel, 'text' | 'done'>>,
+  ) {
+    const set = Object.fromEntries(
+      Object.entries(changes).map(([field, value]) => [
+        `checklist.$.${field}`,
+        value,
+      ]),
+    );
+    if (Object.keys(set).length === 0) return;
+    await this.issues.updateOne(
+      { _id: issueId, 'checklist.id': itemId },
+      { $set: set },
+    );
+  }
+
+  async removeChecklistItem(issueId: string, itemId: string) {
+    await this.issues.updateOne(
+      { _id: issueId },
+      { $pull: { checklist: { id: itemId } } },
+    );
+  }
+
+  async renameParent(parentId: string, title: string) {
+    await this.issues.updateMany(
+      { 'parent.id': parentId },
+      { $set: { 'parent.title': title } },
+    );
+  }
+
   async addAttachment(issueId: string, attachment: AttachmentReadModel) {
     await this.issues.updateOne(
       { _id: issueId, 'attachments.id': { $ne: attachment.id } },