0324 — delivery_log_<topic_id> records only failed attempts; a row's absence is the success signal

Context. An attempt audit trail was wanted, but nothing on a hot path should pay for it, and successes vastly outnumber failures.

Decision. A per-topic delivery_log_<topic_id> table (consumer_group, message_id, attempt, attempted_at, error) records only FAILED attempts, written in the same transaction/statement as the delivery_<topic_id> mutation it shadows. A row’s absence for a given attempt IS the “succeeded” signal, so nothing on any hot path ever reads the table. Retries append distinct rows rather than overwriting, and both retention paths drain it.

Consequences. Success costs zero audit writes; the log is pure history, never control flow. As first built, DisableDeliveryLog skipped creating the table entirely; that part was later replaced — the table is now always created and the flag gates only the writes, so every topic shares one schema shape.