Add GPU type information to health check response

- Updated the health check endpoint to include the type of GPU available (CUDA or MPS).
- Modified the HealthResponse model to accommodate the new gpu_type field, enhancing the response with detailed GPU information.
- This change improves the clarity of system capabilities for users and developers.

backend/models.py

@@ -118,6 +118,7 @@ class HealthResponse(BaseModel):
     model_downloaded: Optional[bool] = None  # Whether model is cached/downloaded
     model_size: Optional[str] = None  # Current model size if loaded
     gpu_available: bool
+    gpu_type: Optional[str] = None  # GPU type (CUDA, MPS, or None)
     vram_used_mb: Optional[float] = None