* 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
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