docs: define CalDAV secret retirement

docs/CALENDAR_INTEGRATION_CONCEPT.md

@@ -69,6 +69,12 @@ bindings or unresolved delivery. Credential rotation does not discard committed
 desired state. Public event mutation cannot set sync-owned source hrefs, kinds,
 or ETags. Deleting a synchronized collection or retiring its source is a local
 unlink: it never deletes the remote collection or its remaining remote events.
+The unlink immediately scrubs Calendar-owned credential ciphertext and external
+provider references and audits the deletion. External provider failure blocks
+retirement before queued work is changed; a later retry tolerates a provider
+secret already removed by an earlier attempt whose database transaction rolled
+back. Provider errors and audit details never contain credential values or
+secret references.
 The current singular ownership model permits one active sync source per
 calendar; multi-source fan-in will require per-event source routing. Celery beat
 triggers recovery every minute, while root-transaction after-commit dispatch