# Firebase Cloud Messaging

## Objective

Deliver push through Firebase with registration tokens that stay valid as devices change.

A Firebase-backed push channel with server-side token lifecycle management, consistent payload handling across platforms, and safe handling of the routes a notification opens.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Store one record per device registration, tied to the account signed in at the time, the platform it came from, and when it was last confirmed working, and let one account hold many of them.
2. Send from the app's existing background-job system with the provider credentials held server-side, and treat every send as a job that can be retried rather than work done inline during a request.
3. Decide, per notification type, whether the payload is meant to be displayed by the platform or handled by the app, and keep that decision the same on every platform. The same event behaving as a visible alert on one platform and a silent update on another produces bugs nobody can reproduce.
4. Treat the destination carried in a notification as untrusted input. Resolve it to a route on open and re-authorise the recipient against the current record before showing anything.
5. If the app also carries the OneSignal push brief, do not run both as independent senders. One of them owns registration, preference filtering, and deduplication for the whole app, and the other is removed or reduced to a delivery transport underneath it.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- The provider will reject tokens that have been rotated, revoked, or belong to a reinstalled app. Remove or replace those registrations on the error rather than leaving them to fail on every future send.
- One person may use several devices, and one device may be used by several people. Keep registrations distinct per device, and reassign rather than duplicate when a new account signs in on a device already registered.
- A notification tapped hours later may point at a record that has since been deleted, moved, or revoked. The open path must check access at that moment and fall back to a sensible screen instead of trusting the link it was given.
- Retried background jobs and repeated provider callbacks must not produce a second notification. Key each send on the event and the registration so a replay is recognised and discarded.
- Send failures come in two kinds: permanent ones that mean the registration is dead, and temporary ones that mean try again later. Retry only the second kind, with increasing delays, or a provider outage will empty the device table.
- Lock-screen previews expose whatever is in the payload. Keep sensitive detail out of the visible portion and let the app fetch it after the user has unlocked and authenticated.
- When the provider is unavailable, the rest of the app must continue working and the user should still see the event in the app's own notification surface.
- Time-sensitive notifications need an expiry so a queue drained after an outage does not deliver a batch of stale alerts at once.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Registrations are stored per device with the owning account and platform, and many per account are supported.
- [ ] Tokens rejected as permanently invalid are removed on the failure, while temporary failures are retried with backoff.
- [ ] A given event and registration pair produces one notification even when jobs or provider callbacks repeat.
- [ ] Display versus silent handling for each notification type is identical across platforms.
- [ ] Opening a notification re-checks permission on the target record before rendering it.
- [ ] Provider credentials exist only on the server and never in client-visible code.
- [ ] A provider outage degrades to in-app notifications with no user-visible errors elsewhere.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.