REFACTOR(secrets): flatten Vault paths

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

grafana/manifests/secret.yaml

@@ -14,9 +14,9 @@ spec:
   data:
     - secretKey: admin-user
       remoteRef:
-        key: monitoring/grafana
+        key: grafana
         property: ADMIN_USER
     - secretKey: admin-password
       remoteRef:
-        key: monitoring/grafana
+        key: grafana
         property: ADMIN_PASSWORD