Add Kubernetes pod disruption budget manifest

## Task
Create a PodDisruptionBudget manifest to ensure high availability during node maintenance.

## Details
- Create `k8s/pdb.yaml`
- Set minAvailable: 1 for the API deployment
- Set maxUnavailable: 0 for critical services
- Add matching labels for the API selector

## Path
- `k8s/pdb.yaml`

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