Implement FHIR R4 topic-based Subscriptions (R5 backport) to receive real-time notifications when specific resource changes occur, using a REST-hook channel

Steps

1. Discover available subscription topics by querying the server's CapabilityStatement or by GETting [base]/SubscriptionTopic to list supported topics
2. Create a Subscription resource via POST to [base]/Subscription specifying the topic URL, channel type (rest-hook), endpoint URL, heartbeat period, and any filter criteria on the topic
3. Respond to the handshake notification sent by the server to the endpoint immediately after subscription creation by returning 200 OK to confirm the endpoint is reachable
4. Process incoming notification bundles of type subscription-notification, extract the notification-event entries, and use the included focus resources or fetch the changed resources via FHIR queries
5. Handle heartbeat notifications (empty bundles) to confirm the subscription is still active, and implement reconnect logic if heartbeats stop arriving within the expected window