The DLQ handler's first real failure

The dead-letter-queue handler had been in place for six months and had never seen a real failure. The first poison message needed a "major fix" twice in one day.

The handler assumed it would see a message once; it saw it three times. It assumed the payload would look like the happy path; it did not. Two changes stuck: the handler became idempotent, and the retry policy became explicit instead of whatever the queue defaulted to.