This PR fixes stale-claim detection by using created_at (immutable) instead of claimed_at (mutable) as the canonical anchor, preventing long-pending tasks from hiding behind recent heartbeats. The change is applied in async_executor.py (_is_stale_claim), store.py (ordering in find_running_by_fingerprint), and models.py (documentation). Corresponding tests verify the new behavior. The logic is sound for both SQLite and Redis stores. Overall, the change is correct and well-tested.