Starting a whisper-server runner loads the gguf onto the GPU, but it was
invisible to the VRAM-eviction logic — it never evicted others to make room,
recorded no footprint, and (lacking a cleanup()) couldn't itself be evicted.

- WhisperServerManager.cleanup() -> stop(), so _evict_one/unload_model can
  free its VRAM like any other model.
- MultiModelManager.start_whisper_server(): estimate the gguf footprint, evict
  other models if free VRAM is short, start the subprocess, and register it in
  models/models_in_vram/_measured_vram_gb (active_in_vram). It's now both a
  trigger for eviction and an eviction candidate.
- stop_whisper_server(): stop + clear all that accounting (frees VRAM).
- Routed every start/stop through these: on-request transcription, engine
  startup pre-load, admin model-load (Load button) and model-unload/disable.

So: starting a runner = a model load (evicts as needed); unloading = frees VRAM.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>