feat(platform): add priority-classes app with low/power/medium/high classes (#174)

## Summary
- New `apps/base/priority-classes/` app with four `PriorityClass` objects managed via the `platform` ArgoCD project
- Adds `apps/overlays/*/priority-classes` to the platform ApplicationSet generator
- Adds `priority-classes` namespace to platform AppProject destinations (required even for cluster-scoped resources)

| Class | Value | PreemptionPolicy | Intent |
|---|---|---|---|
| `low` | 100 | Never | Background work; evictable, won't preempt others |
| `power` | 100 | Never | Compute-heavy but expendable (e.g. AI/ML workloads) |
| `medium` | 10000 | PreemptLowerPriority | Standard services |
| `high` | 100000 | PreemptLowerPriority | Critical services; preempts lower-priority pods |

`PriorityClass` is already in the platform project's `clusterResourceWhitelist` so no project policy changes were needed.

## Test plan
- ArgoCD syncs `platform-priority-classes` successfully
- `kubectl get priorityclasses low power medium high` shows all four classes

Reviewed-on: #174

apps/base/priority-classes/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - priorityclasses.yaml

apps/base/priority-classes/priorityclasses.yaml

@@ -0,0 +1,36 @@
+---
+apiVersion: scheduling.k8s.io/v1
+kind: PriorityClass
+metadata:
+  name: low
+value: 100
+preemptionPolicy: Never
+globalDefault: false
+description: "Low-importance workloads. Can be evicted under pressure but will not preempt other pods."
+---
+apiVersion: scheduling.k8s.io/v1
+kind: PriorityClass
+metadata:
+  name: power
+value: 100
+preemptionPolicy: Never
+globalDefault: false
+description: "Compute-heavy workloads with low scheduling importance. Evictable under pressure."
+---
+apiVersion: scheduling.k8s.io/v1
+kind: PriorityClass
+metadata:
+  name: medium
+value: 10000
+preemptionPolicy: PreemptLowerPriority
+globalDefault: false
+description: "Standard workloads. Will preempt low-priority pods if the cluster is under pressure."
+---
+apiVersion: scheduling.k8s.io/v1
+kind: PriorityClass
+metadata:
+  name: high
+value: 100000
+preemptionPolicy: PreemptLowerPriority
+globalDefault: false
+description: "High-importance services. Will preempt medium- and low-priority pods if necessary."

apps/overlays/au-syd1/priority-classes/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - ../../../base/priority-classes

argocd/applicationsets/platform.yaml

@@ -22,6 +22,7 @@ spec:
       - path: apps/overlays/*/jfrog
       - path: apps/overlays/*/kanidm
       - path: apps/overlays/*/node-feature-discovery
+      - path: apps/overlays/*/priority-classes
       - path: apps/overlays/*/puppet
       - path: apps/overlays/*/purelb
       - path: apps/overlays/*/reflector-system

argocd/projects/platform.yaml

@@ -31,6 +31,8 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'node-feature-discovery'
       server: https://kubernetes.default.svc
+    - namespace: 'priority-classes'
+      server: https://kubernetes.default.svc
     - namespace: 'purelb'
       server: https://kubernetes.default.svc
     - namespace: 'puppet'