fix(handler): add cycle detection to BatchUpdateIssues parent_issue_id handling

BatchUpdateIssues was missing the ancestor-walk cycle detection that
single UpdateIssue has. This allowed creating circular parent
relationships (e.g. A→B→A) via the batch API. Added the same
depth-limited walk (up to 10 ancestors) to detect and skip issues
that would create cycles, consistent with UpdateIssue behavior.

server/internal/handler/issue.go

@@ -1310,6 +1310,23 @@ func (h *Handler) BatchUpdateIssues(w http.ResponseWriter, r *http.Request) {
 				}); err != nil {
 					continue
 				}
+				// Cycle detection: walk up from the new parent to ensure we don't reach this issue.
+				cycleDetected := false
+				cursor := newParentID
+				for depth := 0; depth < 10; depth++ {
+					ancestor, err := h.Queries.GetIssue(r.Context(), cursor)
+					if err != nil || !ancestor.ParentIssueID.Valid {
+						break
+					}
+					if uuidToString(ancestor.ParentIssueID) == issueID {
+						cycleDetected = true
+						break
+					}
+					cursor = ancestor.ParentIssueID
+				}
+				if cycleDetected {
+					continue
+				}
 				params.ParentIssueID = newParentID
 			} else {
 				params.ParentIssueID = pgtype.UUID{Valid: false}