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/pkg/db/queries/issue.sql

@@ -38,6 +38,7 @@ UPDATE issue SET
     assignee_id = sqlc.narg('assignee_id'),
     position = COALESCE(sqlc.narg('position'), position),
     due_date = sqlc.narg('due_date'),
+    parent_issue_id = sqlc.narg('parent_issue_id'),
     updated_at = now()
 WHERE id = $1
 RETURNING *;
@@ -66,3 +67,8 @@ WHERE workspace_id = $1
   AND (sqlc.narg('status')::text IS NULL OR status = sqlc.narg('status'))
   AND (sqlc.narg('priority')::text IS NULL OR priority = sqlc.narg('priority'))
   AND (sqlc.narg('assignee_id')::uuid IS NULL OR assignee_id = sqlc.narg('assignee_id'));
+
+-- name: ListChildIssues :many
+SELECT * FROM issue
+WHERE parent_issue_id = $1
+ORDER BY position ASC, created_at DESC;