0043 — The dead-letter queue is WHERE status='dead', not a separate table

Context. Messages that exhaust their attempts need somewhere to go for inspection and possible replay; brokers typically move them to a separate dead-letter destination.

Decision. Dead rows stay in message_log with status='dead'; the dead-letter queue is the query WHERE status='dead'.

Consequences. No move/copy machinery and no second table to keep in sync — the terminal state is just another value of the same status column, and inspection is a plain SELECT. Dead rows share the table with live traffic, so they occupy the same storage and indexes until cleaned up.