- Add diagnostic logging to track security_header_profile_id conversions
- Replace silent failures with explicit HTTP 400 error responses
- Fix ProxyHostService.Update to properly handle nullable foreign keys
- Fix frontend dropdown to use explicit null checks (no falsy coercion)
- Add 7 comprehensive tests for profile assignment edge cases
Root cause: Backend handler had no else clause for type conversion failures,
causing old values to persist. Fixed by adding logging, error handling, and
changing service layer from Updates() to Select("*") for nullable FKs.
Refs: #<issue_number_if_applicable>
1513c0b636