عرض عملي: سباق بين طلبين على قيد Unique

خطوة بخطوة

1. Implement createProduct as if (!repo.existsBySku(sku)) repo.save(...) with no constraint.
2. Fire two concurrent requests with the same SKU (two threads released by a CountDownLatch).
3. Planned mistake: run it and show two rows with the same SKU.
4. Add V3__unique_sku.sql (clean the duplicate first), then re-run.
5. Map the violation to 409 in the exception handler.
6. Result: one 201 and one 409, every time.