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.

README.md

@@ -7,9 +7,11 @@ Iris pairs with your running `hermes gateway` over a private, token-authenticate
 ## Features
 
 - **Native Hermes-Gateway integration** — your hermes → gateway → Iris app
-- **Absolute Privacy!** — everything stays on your own infrastructure
-  (push: ntfy by default; FCM is opt-in and routes push metadata via Google —
-  see [Push notifications](#push-notifications))
+- **Absolute Privacy!** — chat stays on your own infrastructure
+  (push: ntfy by default, **but the default ntfy server is the public
+  `ntfy.sh`** — self-host ntfy to keep push metadata on your own machine;
+  FCM is opt-in and routes push metadata via Google — see
+  [Push notifications](#push-notifications))
 - **100 MB file uploads by default** — configurable on the gateway via
   `max_upload_bytes` (see [Media](docs/07-media.md) §7.7); all limits are set
   on the gateway side (hermes), not in the app
@@ -48,9 +50,11 @@ hermes-agent ──> hermes gateway ──(HTTP :8791)──> Iris app (Android
 Push wakes a backgrounded/offline device; on reconnect the app syncs the
 outbox, so nothing is lost.
 
-- **ntfy (default)** — push metadata stays on your own infrastructure
-  (self-hosted ntfy recommended). This is the backend for truly private
-  communication.
+- **ntfy (default)** — the backend for truly private communication.
+  ⚠️ **By default it uses the public `https://ntfy.sh` cloud service** — push
+  metadata (topic, notification title) passes through ntfy.sh's servers.
+  Set `NTFY_SERVER_URL` to a **self-hosted ntfy** to keep push metadata on
+  your own infrastructure (recommended; public ntfy.sh SSE is also flaky).
 - **FCM (opt-in, `IRIS_PUSH_BACKEND=fcm`)** — standard/reliable, but FCM push
   metadata (notification title, device token) is routed through **Google's
   servers**. If you want truly private communication, use ntfy instead.