Plan a mobile backend architecture

Architect the server-side backend for a [Mobile App Type] expected to serve [N] active users. Address each of the following mobile-specific challenges: designing an API that minimises data transfer for low-bandwidth and metered connections, building a reliable push notification delivery pipeline with per-platform adapters for APNs and FCM, implementing an offline-first sync strategy with conflict resolution for data modified while the device was offline, handling access token refresh transparently without requiring the user to re-authenticate, and versioning the API to simultaneously support multiple app versions that may lag months behind the latest release.