Docs: clarify ntfy privacy — default server is public ntfy.sh

The docs claimed ntfy keeps push metadata on your own infrastructure,
but the default NTFY_SERVER_URL is the public https://ntfy.sh cloud
service. Make explicit that push metadata (topic, notification title)
passes through ntfy.sh unless you self-host ntfy.

docs/08-push.md

@@ -3,8 +3,11 @@
 The gateway can't reach a sleeping phone directly. Push goes through a cloud
 relay. **Decision: ntfy default, FCM optional** (`IRIS_PUSH_BACKEND`).
 Privacy: FCM push metadata (notification title, device token) is routed
-through Google's servers — for truly private communication use ntfy
-(self-hosted), which keeps everything on your own infrastructure.
+through Google's servers. ntfy is the private option — **but note the default
+`NTFY_SERVER_URL` is the public `https://ntfy.sh` cloud service**, so push
+metadata passes through ntfy.sh's servers unless you self-host ntfy (set
+`NTFY_SERVER_URL`); only a self-hosted ntfy keeps everything on your own
+infrastructure.
 
 ## 8.1 When push fires