matrix, zabbix: add volumeMode to postgres PVC templates

Same ArgoCD + SSA self-heal loop pattern as guacamole (20e4130):
K8s defaults volumeMode=Filesystem on volumeClaimTemplates at
creation, git omits it, argocd-controller owns the atomic list so
every reconcile sees drift, and volumeClaimTemplates is immutable
so it can never reconcile. Adding the field closes both loops.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/matrix/matrix.yaml

@@ -137,6 +137,7 @@ spec:
         name: matrix-postgres-data
       spec:
         accessModes: [ReadWriteOnce]
+        volumeMode: Filesystem
         resources:
           requests:
             storage: 5Gi