REFACTOR(secrets): flatten Vault paths

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

alertmanager/manifests/secret.yaml

@@ -14,7 +14,7 @@ spec:
   data:
     - secretKey: smtp_auth_password
       remoteRef:
-        key: monitoring/alertmanager
+        key: alertmanager
         property: SMTP_PASSWORD
 ---
 apiVersion: external-secrets.io/v1
@@ -81,5 +81,5 @@ spec:
   data:
     - secretKey: smtp_password
       remoteRef:
-        key: monitoring/alertmanager
+        key: alertmanager
         property: SMTP_PASSWORD