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

server/cmd/server/router.go

@@ -177,6 +177,7 @@ func NewRouter(pool *pgxpool.Pool, hub *realtime.Hub, bus *events.Bus) chi.Route
 					r.Post("/reactions", h.AddIssueReaction)
 					r.Delete("/reactions", h.RemoveIssueReaction)
 					r.Get("/attachments", h.ListAttachments)
+					r.Get("/children", h.ListChildIssues)
 				})
 			})