model-load/model-unload were proxied to the primary engine, so unloading
(or loading) a model that lives on a secondary engine hit the wrong process
and silently no-op'd (was_loaded=False). Add front-proxy interceptors:

- unload: find the engine whose loaded_models matches the path and forward
  the request there; fall back to the primary.
- load: reuse an engine already serving the model, else the model's engine
  pin from models.json, else the primary.

Registered before the catch-all proxy, mirroring /admin/api/engines.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>