Model a whisper gguf as two things: a MODEL config (a .gguf entry with
backend=whisper-server and NO model_path — enables the model, holds load
strategy, shown on the GGUF row) and a RUNNER (backend=whisper-server WITH
model_path — the subprocess, shown in the whisper card).

- Enabling a .gguf with speech_to_text marks it backend=whisper-server and
  auto-creates exactly one runner (1:1) on a free port.
- Disabling the model removes + kills all its runners (cascade by model_path).
- Removing a runner (or model) now stops the subprocess + drops registry
  entries, instead of leaving it running until restart.
- cached-models shows the model config on the GGUF row but excludes runners;
  the whisper card shows only runners (require model_path).
- engine startup only launches runners (entries with model_path), never the
  bare model config.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>