feat(woodpecker): set workflow pod priority class to power (#175)

## Summary
Sets `WOODPECKER_BACKEND_K8S_PRIORITY_CLASS: power` on the Woodpecker agent so all CI pipeline pods are scheduled with the `power` PriorityClass (value 100, preemptionPolicy: Never).

This means pipeline pods can be evicted when the cluster is under pressure but won't preempt other workloads.

## Dependency
Requires the `power` PriorityClass to exist on the cluster — deploy PR #174 (priority-classes app) first.

## Test plan
- Trigger a pipeline run and confirm pods are created with `priorityClassName: power`
- `kubectl get pod -n woodpecker -o jsonpath='{.items[*].spec.priorityClassName}'`

Reviewed-on: #175

apps/overlays/au-syd1/woodpecker/values.yaml

@@ -2,6 +2,7 @@ agent:
   replicaCount: 3
   env:
     WOODPECKER_MAX_WORKFLOWS: "8"
+    WOODPECKER_BACKEND_K8S_PRIORITY_CLASS: power
     WOODPECKER_BACKEND_K8S_STORAGE_CLASS: cephrbd-fast-delete
     WOODPECKER_BACKEND_K8S_VOLUME_SIZE: 10G
     WOODPECKER_BACKEND_K8S_STORAGE_RWX: false