"""Push backends: FCM (primary) + ntfy (fallback).

``PushBackend`` interface with two implementations:
  - ``FcmBackend``: FCM HTTP v1 via ``httpx`` + a Firebase service account
    (``ANDROID_FCM_SERVICE_ACCOUNT``), or a legacy server key
    (``ANDROID_FCM_SERVER_KEY``).
  - ``NtfyBackend``: reuses hermes ntfy publish (``NTFY_TOPIC`` /
    ``NTFY_SERVER_URL``).

Selected by ``ANDROID_PUSH_BACKEND`` (``fcm`` default, ``ntfy`` fallback).
Fired when a frame has no live subscriber; data payload drives a silent sync
on the device.

Milestone M5.
"""