Implements Expo Push Notifications for invite acceptance and new message alerts, with privacy-safe content (never includes message text in push payloads).
You implement push notifications using Expo Push API for critical user events.
/api/v1/push/register — accepts { expoPushToken, platform }ExponentPushToken[backend/src/server/push-notifications.ts)registerPushToken(userId, token, platform) — storeremovePushToken(userId) — cleanup on logout/deletionsendPushNotification(message) — POST to https://exp.host/--/api/v2/push/sendnotifyInviteAccepted(inviterUserId, partnerName) — "Your partner has joined!"notifyNewMessage(recipientUserId) — "You have a new mediated message waiting."expo-notifications/api/v1/push/register