0187 — No ON CONFLICT on the exception INSERT

Context. The plan suggested ON CONFLICT DO UPDATE on the deliveries INSERT that records failures, on the assumption a row might already exist.

Decision. Plain INSERT, no conflict clause. Tracing showed a collision cannot happen in this design: a message_id belongs to exactly one range ever (claimed only moves forward), and because Commit frees the lease before inserting (see 0182), only the still-owning worker ever reaches the INSERT.

Consequences. A real primary-key violation now surfaces loudly as an error instead of being silently absorbed by defensive SQL — if the invariant ever breaks, it is visible. Rejected: ON CONFLICT DO UPDATE — defensive handling for a case with no actual trigger, which would have masked invariant violations.