Email and push notification strategy

## Summary
Define and implement the product strategy for email and push notifications in a self-hosted RFChat deployment.

## Scope
- Decide which notification channels are supported first and what product behaviors they cover.
- Define user preferences, delivery expectations, and failure/fallback behavior.
- Align notification delivery with unread, mention, and presence semantics that users see in the product.
- Make operator responsibilities explicit for delivery infrastructure.

## Acceptance Criteria
- [ ] Supported notification channels and their triggers are explicitly defined.
- [ ] User preference controls match the actual delivery capabilities of the deployment.
- [ ] Failed or unavailable delivery paths do not imply success to users.
- [ ] Operators have a documented configuration story for the chosen delivery channels.

## Operational Notes
- Self-hosted deployments vary widely; the strategy must distinguish optional integrations from built-in guarantees.
- This issue is about a real deployable plan, not placeholder toggles for channels that do not exist.