Subscription Processing

**User story**
As a client, I want Subscriptions to push notifications to their specified `channel` (`rest-hook`/`websocket`/`email`/`sms`/`message`) based on their specified `criteria`. Retries, error handling, and `status` changes should be applied with sane defaults.

https://hl7.org/fhir/R4/subscription.html

**Acceptance criteria**
1. For an active Subscription, changes to resources that meet the Subscription criteria trigger a notification via the specified channel.
2. Failed notifications should be retried with sane defaults.
2. Undeliverable notifications should trigger a change in Subscription status to `error`.

Start with the linked FHIR R4 Subscription specification and trace the server's existing handling of Subscription criteria, channels, retries, and status changes. Done means active subscriptions notify through the specified channel, failed deliveries retry with defined defaults, and undeliverable notifications set the Subscription status to error.

Written by the indexing model from the issue text.