Recover Transient Dead Letters

Demote DEAD_LETTER episodes whose failure was an infra transient.

The state-machine rule before the mark_transient_failure fix burned an attempt on every OperationalError(AdminShutdown) the extractor caught during savepoint release, so a 4-reconnect burst on a managed Postgres would terminally retire a healthy chat (chat f9ebb5b1 on 2026-05-28 was the prompting incident). The forward fix prevents new occurrences; this endpoint clears the backlog.

Bounded to LIMIT 100 per call so a misclassified pattern can't storm the whole table. Demotes status DEAD_LETTER → FAILED and zeroes attempts so the sweep's normal retry budget applies. Operates only on rows whose stored failure_reason matches a transient-class prefix - logical failures stay terminal.