refactor: update Vault secret paths to new categorized structure

- alertmanager: alertmanager → observability/alertmanager
- grafana: postgresql → storage/postgresql
- prometheus: postgresql → storage/postgresql, minio → storage/minio
- thanos: minio → storage/minio

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

grafana/manifests/secret.yaml

@@ -14,5 +14,5 @@ spec:
   data:
     - secretKey: password
       remoteRef:
-        key: postgresql
+        key: storage/postgresql
         property: PASSWORD