Plan Push Notifications

Background: Native [iOS / Android / both] app. I want to send these kinds
of notifications: [e.g., transactional alerts, marketing, silent/data
pushes for sync]. Backend: [chosen one].

Use case: Plan the end-to-end push notification setup and the client-side
handling.

Implementation details:
- iOS: APNs (directly or via Firebase Cloud Messaging), permission
  request timing, provisional authorization, notification service/content
  extensions if I need rich pushes
- Android: FCM, the POST_NOTIFICATIONS runtime permission (Android 13+),
  notification channels, and Doze/background limits
- Token registration and refresh, and sending the token to my backend
- Handling taps that deep-link into a specific screen
- Silent/background pushes and their platform limits

Layer & dependencies: Permission prompts should be requested in context
(after showing value), not on first launch.

Deliverable: An end-to-end plan (device registration → backend storage →
send → receive → tap handling), the platform-specific gotchas, a good
permission-priming strategy, and the data model for storing device
tokens.