Expose logging and secrets_detection in /info, remove unused log_content

Expose logging and secrets_detection in /info, remove unused log_content

Follow-ups from #91:

- /info now includes secrets_detection and logging sections so users can
  verify their config is loaded (the issue reporter could not).
- The dashboard fallback message no longer claims log_masked_content is
  false when content is simply absent for another reason.
- Remove the log_content option: it was defined in config and documented,
  but no code ever read it. Logging raw request/response content would
  contradict the privacy guarantees, so it is removed rather than
  implemented. Zod strips unknown keys, so existing configs that still
  set log_content keep loading.