Implement AUDIT-WP-0006 honest operational custody.

Postgres now reports custody_class=operational with a cited 30-day
recoverable window. Join ITC-CAP operations.audit at D4, publish the
interface card, and overlay user-engine tenants [*] from Git so an
ExternalSecret refresh cannot shrink it.

audit_core/__init__.py

@@ -13,6 +13,7 @@ from audit_core.interface import (
     IdempotentAuditBackend,
     RetentionPolicy,
     SCHEMA_VERSION_V1ALPHA1,
+    custody_class_satisfies,
     validate_event,
 )
 from audit_core.mock_file_backend import MockFileAuditBackend
@@ -30,5 +31,6 @@ __all__ = [
     "RetentionPolicy",
     "SCHEMA_VERSION_V1ALPHA1",
     "SQLiteAuditBackend",
+    "custody_class_satisfies",
     "validate_event",
 ]