Concurrent changes and retries

• Stale writes are refused. Issues, documents, sprints and the other editable records carry a version; a change made against an outdated one is refused with the current state. Since 2026.10.23 so are the workflow and the Definition of Done. The other whole-configuration replacements (working calendar, section models, status definitions, a repository connection, instance settings) are last-write-wins, each declared with its reason.
• Retries do not repeat. Every write that would repeat an effect accepts an idempotency key; a retry with the same key within 24 hours gets the first answer, and the same key with a different request is refused. The creates that return a secret take no key. After 24 hours a key is a new request, and its stored answer is deleted.
• A busy database says so (since 2026.11.0). When no database connection frees up in time, the request is answered 503 «Database busy» with a Retry-After, never a generic error, so a client knows to retry.