TransferGiftCard locked source-then-destination in caller-chosen order, so concurrent cross-transfers (A→B and B→A) acquired row locks in opposite orders and deadlocked (SQLSTATE 40P01), aborting one transfer with a generic 500. Both rows are now locked in lexicographically sorted ID order (case-insensitive, matching the 12-hex mixed-case card codes), with the scanned values mapped back to source/destination roles afterward. Adds a deterministic concurrency regression test that holds both row locks on dedicated connections to force the AB-BA cycle, proving the old code deadlocks and the new code serializes cleanly.
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