0184 — ClaimExceptions dead-letters crash-looping rows before claiming

Context. An exception whose processing crashes the worker never reaches RecordExceptionFailure, so it can never resolve itself — without a backstop it would be re-claimed on expiry forever.

Decision. ClaimExceptions runs a kill backstop first: expired-inflight rows already at maxAttempts are moved to dead with no user code involved. Only then does it claim ready and expired-inflight rows into inflight, joined to message_log for the payload.

Consequences. The backstop is the only way a poison exception ever leaves the retry loop — placing it inside the claim query means no separate reaper process is needed, and a crash-looping message terminates at dead after its attempt budget exactly like an ordinary exhausted failure.