Use FHIR subscriptions to receive webhook notifications when patient data changes

Verified steps

1. Check the server's CapabilityStatement for subscription support; in FHIR R4 use the Subscription resource; in FHIR R4B/R5 the SubscriptionTopic resource provides more structured event definitions.
2. Create a Subscription resource via POST [base]/Subscription with criteria specifying the resource type and optional filter (e.g., 'Observation?patient=[id]&code=http://loinc.org|[code]'), channel.type of 'rest-hook', and channel.endpoint set to your HTTPS webhook URL.
3. Include channel.header with an Authorization header value your webhook endpoint will validate to authenticate incoming notifications from the FHIR server.
4. The server sends HTTP POST notifications to your endpoint when matching resources change; the notification body may be a minimal ping (empty Bundle), the changed resource, or a full payload depending on server configuration and Subscription.channel.payload setting.
5. Respond to webhook notifications with HTTP 200 within a few seconds; if processing takes longer, acknowledge immediately and process asynchronously to avoid notification retries.
6. Handle subscription lifecycle: check Subscription.status (requested → active → error); the server may transition to 'error' if your endpoint is unreachable; re-create or reactivate subscriptions after downtime.