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/README.md

@@ -30,7 +30,7 @@ top-to-bottom once, then use the numbered docs as a lookup while implementing.
 | 5 | [`05-streaming.md`](05-streaming.md) | Streaming / reasoning / tools / intermediate. |
 | 6 | [`06-channels-cron-search.md`](06-channels-cron-search.md) | Channels, threads, cron delivery, search. |
 | 7 | [`07-media.md`](07-media.md) | Media upload/download + playback. |
-| 8 | [`08-push.md`](08-push.md) | Push (FCM + ntfy), outbox, sync. |
+| 8 | [`08-push.md`](08-push.md) | Push (ntfy default + FCM optional), outbox, sync. |
 | 9 | [`09-pairing-security.md`](09-pairing-security.md) | Pairing, auth, security model. |
 | 10 | [`10-android-app.md`](10-android-app.md) | When building the Android app. |
 | 11 | [`11-desktop-app.md`](11-desktop-app.md) | When building the Desktop app. |
@@ -47,6 +47,7 @@ Machine-readable / diagrams:
 
 - [`protocol/frames.schema.json`](protocol/frames.schema.json) — wire-frame schema.
 - [`diagrams/architecture.mmd`](diagrams/architecture.mmd) — mermaid architecture.
+- [`playstore-listing.md`](playstore-listing.md) — Play Store listing text (incl. the FCM/ntfy privacy note).
 
 ---