Backend inference interface: accept a CAM output from the swin classifier

The FastAPI EigenGradCAM service (ai-cfia/nachet-model-ccds#38) returns a second
Triton output, `CAM` (FP32, shape `[images, k, grid, grid]`), alongside the usual
`PREDICTIONS`.

The backend can't consume this today. `TritonInferenceResponse` validates that a
swin response has exactly one output named `PREDICTIONS`:

- `backend/app/model/inference.py:609` — `raise ValueError("27spp model returns exactly one output (PREDICTIONS)")`
- `backend/app/model/inference.py:582` — output name must be `"PREDICTIONS"`

To surface explainability through the platform, the backend response contract needs
to optionally accept and pass through a `CAM` output when the model emits one, without
breaking models that return predictions only.

Scope:
- [ ] Relax/extend the swin response model to allow an optional `CAM` output
- [ ] Thread the CAM data through to the API response the frontend consumes
- [ ] Note: `CAM.data` is a nested FP32 array, not a JSON string (unlike PREDICTIONS)

Blocks the "Update Nachet Backend" step of ai-cfia/nachet#34.
Ref: ai-cfia/nachet-model-ccds#38.