docs: Notifications architecture & channel setup

[joelpeace48-cell]

Background & current state

Notifications span channels (in-app NEW-094, web push NEW-093, email #342) + preferences (NEW-096), but there's no doc on configuring them.

Goal

Author docs/NOTIFICATIONS.md: how to configure each channel + the preferences model.

Outline

• Channels: in-app, web push (VAPID), email; setup/config per channel.
• Preferences model + enforcement (NEW-096); event taxonomy; critical (non-disableable) notices.
• Delivery/troubleshooting.

Task breakdown

• Write docs/NOTIFICATIONS.md with per-channel setup.
• Document the event taxonomy + preferences.
• Cross-link.

Acceptance criteria

• Each channel has clear setup steps; preference behavior documented.

Verification

• Follow setup in staging; review.

Difficulty: easy · Effort: S–M · documentation