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/facades/issue.facade.ts

@@ -26,6 +26,11 @@ export class IssueFacade {
   readonly history = this.store.history;
   readonly outwardLinks = this.store.outwardLinks;
   readonly inwardLinks = this.store.inwardLinks;
+  /** Tickets hanging from the open one, and how far they have got. */
+  readonly children = this.store.children;
+  readonly childProgress = this.store.childProgress;
+  /** Ticked steps of the open ticket's checklist. */
+  readonly checklistProgress = this.store.checklistProgress;
   readonly loading = this.store.loading;
   readonly error = this.store.error;
   /** Signed-in user's id (authors can edit their comments). */
@@ -94,6 +99,47 @@ export class IssueFacade {
     );
   }
 
+  /** Places the open ticket under another one by its key; null: back to the top. */
+  setParent(parentKey: string | null): void {
+    this.withIssue((issueId) =>
+      this.dispatch.parentRequested({ issueId, parentKey }),
+    );
+  }
+
+  /** Creates a ticket under the open one. */
+  addSubtask(title: string, type: string): void {
+    const issue = this.store.issue();
+    if (!issue || !title.trim()) return;
+    this.dispatch.subtaskCreateRequested({
+      projectId: issue.projectId,
+      parentId: issue.id,
+      type,
+      title: title.trim(),
+    });
+  }
+
+  addChecklistItem(text: string): void {
+    if (!text.trim()) return;
+    this.withIssue((issueId) =>
+      this.dispatch.checklistAddRequested({ issueId, text: text.trim() }),
+    );
+  }
+
+  updateChecklistItem(
+    itemId: string,
+    changes: { text?: string; done?: boolean },
+  ): void {
+    this.withIssue((issueId) =>
+      this.dispatch.checklistUpdateRequested({ issueId, itemId, changes }),
+    );
+  }
+
+  removeChecklistItem(itemId: string): void {
+    this.withIssue((issueId) =>
+      this.dispatch.checklistRemoveRequested({ issueId, itemId }),
+    );
+  }
+
   /** Attaches files to the ticket (or under one of its comments). */
   attach(files: File[], commentId: string | null = null): void {
     if (!files.length) return;