docs: note multi-backend shared-Redis config in README tutorial

The hand-holding tutorial in README §3.6 didn't mention that with
multiple backends behind a load balancer, you only need to configure
on any one node — all backends share one Redis. deployment.md §10.2
covers this, but the tutorial is what users follow during setup.

README.md

@@ -196,6 +196,10 @@ Then, in order:
 Secrets are write-only: the page never shows a stored secret again, and submitting an empty secret
 field keeps the previously stored value — clear a secret deliberately in a separate edit.
 
+If you run multiple backends behind a load balancer, configure on **any one node** — all backends
+share one Redis and read the same configuration. See [the multi-instance section of the deployment
+guide](docs/deployment.md#102-multi-instance-prerequisites) for the full model.
+
 ### 3.7 Register the callbacks
 
 Still on the same page, under **External callbacks**:

README.zh-CN.md

@@ -138,6 +138,8 @@ curl localhost:8080/healthz   # → ok
 
 密钥是只写的：页面永不再显示已存密钥，提交空的密钥字段会保留之前的值——要清密钥就在单独一次编辑里故意留空。
 
+如果你在负载均衡器后面跑了多个后端，**在任意一台节点上配置即可**——所有后端共享同一个 Redis，读同一份配置。完整的多实例模型见[部署文档的多实例章节](docs/deployment.zh-CN.md#102-多实例前提)。
+
 ### 3.7 注册回调
 
 仍在同一页，**External callbacks** 区：