feat(agent): add durable existing-agent credential transitions

backend/tests/test_agent_registry_postgres.py

@@ -73,7 +73,8 @@ class AgentRegistryPostgresTests(unittest.TestCase):
                         version,
                         ip_address,
                         agent_secret_hash,
-                        credential_created_at
+                        credential_created_at,
+                        legacy_auth_disabled_at
                     )
                     VALUES (
                         %s,
@@ -85,6 +86,7 @@ class AgentRegistryPostgresTests(unittest.TestCase):
                         %s,
                         %s,
                         %s,
+                        now(),
                         now()
                     )
                     """,