Batch reference idempotency works as specified: replaying the same gateway sync payload
does not create a second batch row or re-process observations. This is the clinical
equivalent of POS offline sync deduplication and protects against network retries after
partial ACK. The integration test `DuplicateBatchReference_ReturnsExisting` passed in
702 ms, giving CI-guaranteed evidence for interview demos without requiring a manual
chaos run.
