Add Kubernetes horizontal pod autoscaler manifest

## Task
Create a HorizontalPodAutoscaler manifest for auto-scaling the API.

## Details
- Create `k8s/hpa.yaml`
- Target CPU utilization: 70%
- Min replicas: 2, Max replicas: 10
- Reference the API Deployment
- Add memory utilization target as secondary metric

## Path
- `k8s/hpa.yaml`

## Test
Run `kubectl apply --dry-run=client -f k8s/hpa.yaml` to validate.