Correct SES diagnostic; iPartner transactional now isolated

Correction: the earlier claim that no code passes a configuration set was wrong. It grepped only lib/ses.ts (the client singleton) — newsletter.org sends from lib/issues.ts, which does pass ConfigurationSetName and sets both List-Unsubscribe headers. That track was already correct. The real gap was iPartner's transactional path, now fixed: new ipartner-transactional config set (reputation metrics, suppression, TlsPolicy=REQUIRE) with an SNS event destination, and ConfigurationSetName threaded through sendEmail(). Verified by live send and by falsification — a bogus config set raises ConfigurationSetDoesNotExistException. Still unverified: whether anything subscribes to the SNS topic. The IAM user lacks SNS:ListSubscriptions, so events may still be going nowhere.