Track topic subscriptions locally

[xtrinch]

Summary

Add optional local topic subscription tracking so Django can query topic memberships for devices managed through fcm-django.

Why

Firebase's Admin SDK supports subscribing and unsubscribing registration tokens to topics, but it does not expose an API to list topic memberships for a token. This package therefore had no way to answer the request from discussion #263.

What changed

• add TRACK_TOPIC_SUBSCRIPTIONS setting
• add FCMDeviceTopic model and migration
• sync local topic membership on successful subscribe/unsubscribe calls made through fcm-django
• add queryset and instance helpers for reading tracked topic memberships
• document the local-tracking limitation and add test coverage for default and swapped device models

Impact

Projects can opt into local topic tracking and query topic membership from Django, while leaving the current behavior unchanged by default.