address review round 2: gate zero-credit log behind DB auth, surface IP in message body, isolate snips row assertions

- the zero-credit canonical log now sits behind the same USE_DB_AUTHENTICATION
  gate as billable rows, matching the PR's privacy claim exactly
- the client IP rides in the message body too, so non-production console
  transports (which only serialize metadata for warn/error) still show it
- snips snapshot the max keyless_credit_usage id and assert no rows past it,
  isolating against rows accumulated by pre-log-only runs of these tests