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/00-overview.md

@@ -57,7 +57,7 @@ Everything in the feature checklist below.
 | Decision | Choice |
 |---|---|
 | Desktop app tech | **Compose Multiplatform** (shares Android code; "tweaked" for big screen) |
-| Push backend | **Both** — FCM primary, ntfy fallback (`IRIS_PUSH_BACKEND`) |
+| Push backend | **Both** — ntfy default, FCM optional (`IRIS_PUSH_BACKEND`) |
 | Media transport | **Over the WebSocket** (chunked binary frames; no extra Python deps) |
 | Phone default layout | **User-toggleable, single-pane default** (auto two-pane on large screens) |