Two whisper-server UI issues on a multi-engine node:

- Unload didn't visibly take effect: a whisper-server is registered both as
  a subprocess (whisper_servers) AND in the generic .models/.model_pools
  registry under audio:<id>. The unload stopped the subprocess but left the
  registry entry, so the row kept showing "Unload". Now it also drops the
  audio:<id>/<id> registry entries (and matches by id, audio:<id>, or the
  gguf model_path, so unloading the file stops every server using it).

- The backing gguf file showed "Load" while its whisper-server was running.
  Surface each running server's _model_path in the loaded-key sets
  (engine-state, model-loaded-status, status) so the GGUF-file row reflects
  that the file is in use.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>