Incremental Code Review Summary

The latest PR head matches the previously reviewed implementation, including the allow-listed first-party app slug resolution and the fixes for custom-app mailbox generation, warning-cache growth, and tenant sender logging. The existing first-party sender-domain issue remains open and unchanged, so it was intentionally not reposted.

The parallel review workers were interrupted before returning findings; no new actionable findings are available from this increment. A summary review is being recorded with no new comments.

🧪 Browser testing: Skipped — PR only modifies backend authentication email rendering and delivery, with no user-facing browser UI impact.