observability: scrape CloudNativePG postgres metrics (VMPodScrape)

Follow-up to #239. Every CNPG instance pod exposes metrics on :9187 but
nothing scraped them. Add one namespace-wide VMPodScrape (namespaceSelector
any, selector cnpg.io/podRole=instance) so the VMAgent collects postgres
metrics for all ~23 CNPG clusters (authentik, grafana, woodpecker, etc.).

apps/base/observability/vmpodscrape-cnpg.yaml

@@ -0,0 +1,17 @@
+---
+# Scrape metrics (:9187) from every CloudNativePG postgres instance pod in any
+# namespace. Picked up by the observability VMAgent (selectAllByDefault).
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMPodScrape
+metadata:
+  name: cnpg-clusters
+  namespace: observability
+spec:
+  namespaceSelector:
+    any: true
+  selector:
+    matchLabels:
+      cnpg.io/podRole: instance
+  podMetricsEndpoints:
+    - port: metrics
+      path: /metrics