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/12-toolchain.md

@@ -77,15 +77,16 @@ hermes --version       # sanity
 4. The app's `FirebaseMessagingService` obtains the FCM token at runtime and
    registers it via `hello` / `fcm.register`.
 
-> Skip Firebase → set `IRIS_PUSH_BACKEND=ntfy` and configure `NTFY_TOPIC` /
-> `NTFY_SERVER_URL` (self-host ntfy or use ntfy.sh). See `08-push.md`.
+> Skip Firebase → the default is already ntfy: leave `IRIS_PUSH_BACKEND` unset
+> (or set it to `ntfy`) and configure `NTFY_TOPIC` / `NTFY_SERVER_URL`
+> (self-host ntfy or use ntfy.sh). See `08-push.md`.
 
 ## 12.6 Environment variables (summary)
 
 **Secrets (`~/.hermes/.env`):**
 ```
 IRIS_TOKEN=<64-hex>
-IRIS_PUSH_BACKEND=fcm            # or ntfy
+IRIS_PUSH_BACKEND=ntfy           # default; fcm = opt-in (metadata via Google)
 IRIS_FCM_SERVICE_ACCOUNT=/path/to/service-account.json
 # IRIS_FCM_SERVER_KEY=<legacy key>   # fallback if no service account
 # NTFY_TOPIC=iris-push                  # when ntfy