Add optional strategy field to postiz Deployment

Allows overriding the default RollingUpdate via values.strategy — needed
for deployments pinned to a single node by a local-path PVC, where
RollingUpdate can't schedule the surge pod and gets stuck Pending.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0118dTCRPv2onBXbZ3Nb7E2X

charts/postiz/templates/postiz-deployment.yaml

@@ -8,6 +8,10 @@ spec:
   {{- if not .Values.autoscaling.enabled }}
   replicas: {{ .Values.replicaCount }}
   {{- end }}
+  {{- with .Values.strategy }}
+  strategy:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
   selector:
     matchLabels:
       {{- include "postiz.selectorLabels" . | nindent 6 }}