REFACTOR(secrets): flatten Vault paths

- Change secret paths from <category>/<app> to <app>
- monitoring/alertmanager → alertmanager
- monitoring/grafana → grafana
- databases/postgresql → postgresql

prometheus/manifests/secret.yaml

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