feat(issues): add sub-issue support (#483)

* feat(issues): add sub-issue support

- Backend: Add ListChildIssues SQL query, add parent_issue_id to UpdateIssue,
  add GET /api/issues/{id}/children endpoint
- Frontend: Display parent issue breadcrumb and link in issue detail sidebar,
  show child issues list with status icons, add "Create sub-issue" action in
  dropdown menu and sidebar, pass parent_issue_id through create issue modal
- Update test mocks for new API method

* fix(issues): add parent validation, cycle detection, and improve child refresh

- CreateIssue: validate parent issue exists in the same workspace
- UpdateIssue: validate parent exists, prevent self-referencing, detect
  circular parent chains (up to 10 levels deep)
- Frontend: derive child issues from store when available instead of
  refetching on every global issue count change

apps/web/shared/api/client.ts

@@ -196,6 +196,10 @@ export class ApiClient {
     });
   }
 
+  async listChildIssues(id: string): Promise<{ issues: Issue[] }> {
+    return this.fetch(`/api/issues/${id}/children`);
+  }
+
   async deleteIssue(id: string): Promise<void> {
     await this.fetch(`/api/issues/${id}`, { method: "DELETE" });
   }