Default push backend to ntfy; FCM opt-in with privacy warning (issue #10)

- IRIS_PUSH_BACKEND now defaults to ntfy (keeps push metadata on your own
  infrastructure); FCM is opt-in via IRIS_PUSH_BACKEND=fcm
- build_push_backend(): ntfy for empty/unknown names, FCM only on explicit 'fcm'
- gateway setup: warn when FCM is chosen (metadata routed via Google's servers)
- README: privacy note + dedicated push section; new docs/playstore-listing.md
  with the FCM/ntfy privacy note for the Play Store listing
- docs: 00/02/03/08/12/16 + setup.md updated to ntfy-default wording
- tests: default-backend assertion updated (86/86 pass)

docs/playstore-listing.md

@@ -0,0 +1,40 @@
+# Play Store Listing
+
+Copy-paste text for the Play Console (and the F-Droid / sideload pages).
+Keep this file in sync with the README whenever the privacy story changes.
+
+## Short description (≤ 80 chars)
+
+Private chat for your personal AI agent — Android & desktop.
+
+## Full description
+
+Iris is a native chat app for your personal AI agent (hermes-agent):
+streaming replies, visible reasoning, structured tool activity, channels,
+threads, media, search — Telegram-quality, on your own infrastructure.
+
+**Absolute Privacy!** — everything stays on your own infrastructure:
+
+- Your gateway, your machine, your data. No cloud middleman for chat.
+- **Push notifications:** ntfy by default — push metadata stays on your own
+  (self-hosted) ntfy server.
+- **FCM is opt-in** (`IRIS_PUSH_BACKEND=fcm`): standard and reliable, but FCM
+  push metadata (notification title, device token) is routed through
+  **Google's servers**. If you want truly private communication, use ntfy
+  (self-hosted) instead — it is the default.
+
+No file limit. No character limit. Full markdown + HTML artifact preview.
+All settings live in the app.
+
+Runs on Android and desktop (Linux, macOS, Windows) from one shared codebase.
+
+## Privacy note (for the "Data safety" section / FAQ)
+
+Iris talks directly to your own hermes gateway over a private, token-authenticated
+connection. By default, push notifications use ntfy, which you can self-host so
+that push metadata never leaves your infrastructure. If you explicitly enable
+FCM, push metadata (notification title, device token) is sent via Google's FCM
+servers; chat content itself is not sent to Google — FCM only carries a short
+preview, and full content is fetched from your gateway over the authenticated
+connection. For truly private communication, use the default ntfy backend
+(self-hosted).